I’ve previously worked for one of the very big company that prepares all the food and drinks trolleys for flights, and I can safety say that very little you do personally will reduce the waste produced by the flight.

The food carts are not prepared by the airlines. They are a subcontracted service. The number of meals (plus extras) are prepared based on the passenger manifest several hours before takeoff. Special dietary requirements are taken into account, but a full service will be packed for every ticketed passenger plus a few more to account for losses/changes. A meal will be there if your ticket includes it. People change their minds all the time so better to have one and not need it than not have one and end up with a passenger making a scene.

The crew – who are principally there for your safety and not to serve you food and drinks – will dish out the meals. If you decline, it will stay in the cart. But it isn’t their job to sort waste. They have limited time and space. They will collect up everything and pack it back into the carts, stowed away safely in case something goes wrong.

When your flight lands, the carts – now full of waste – will be collected by the contracted company. All cooked food will be disposed of touched or not. Anything that has been opened (including that $100 box of chocolates in first class that no one touched), or anything that has been in any way marked (like wine and champagne bottle labels stained from spills) will be disposed of. The contracting company cannot reuse it. Basically, the only thing that is reused between flights are canned drinks. Fundamentally, it was already paid for by the airline anyway.

The best thing you can personally do is to reduce your own travel weight. Physics doesn’t care about economics. The heavier the plane, the greater the lift needed to keep it in the air, and the more fuel needed to generate the lift. The only way to reduce this is to travel as light as you can.

Bring your own mug and ask them to pour drinks in that. Other answers suggest to bring your own food and drink, but as pointed out by others, that’s not really helping anyone for a variety of reasons.

The mug on the other hand will replace plastic cups which would otherwise be used by you. Those unused cups will just stay on the plane and will be used for future customers (as opposed to the food that will go off and thrown away).

An out of the box option

Whilst most answers here have addressed what to take or not take on a flight, I have a more abstract solution to the problem of reducing waste on the flight, one with much larger gains.

Take a notebook and pen (if at all possible), and then carefully observe for practices on airline flights you take that you consider to be wasteful (inversely, observe what other airlines do right). Different airlines are bound to use different approaches regarding waste. Some will have better policies than others – so if you’re not an ‘ideas person’, you can at least suggest the good policies from one airline, to another one that hasn’t yet implemented it.

What you effectively do is document suggestions for improvements and send those suggestions to the respective airline(s) [either via letter, or email if you’re able to find a suitable address]. If the suggestion(s) are successful, you’d be reducing waste on all flights, and not merely for yourself on your own.

There are a lot of replies to this question and there are bits of an answer spread all over, but the discussion has been dominated by CO2 emissions, rather than addressing the question asked. I’m going to collect the valuable information here and add in some of my own research. Please feel free to improve this answer, it is a community wiki (but you don’t need to add any more about CO2, there are already very well thought out answers that do this).

TL;DR

The best options for reducing your in-flight waste are to use a budget airline with no in-flight service, or declaring days in advance of your flight that you don’t want an in-flight meal on a full-service airline. If waste minimisation is a concern, you should first address the huge CO2 emission cost first, and seriously considering not taking the flight in the first place.

CO2

First and foremost, the largest environmental concern in taking a flight is the CO2 emissions. Using the link provided by Rsf, a 10 hour flight could produce in the order of 900kg of CO2, a significant chunk of the average yearly carbon footprint of a single person. But for the sake of answering the question, we’re only going to look at in-flight waste. Perhaps a large CO2 offset has been purchased to try and mitigate emissions, or perhaps an individual is tracking a CO2 budget and has accounted for the flight.
If you are not at least doing this, then waste minimisation will make a comparatively tiny impact on the environmental cost of your flight.

Waste

According to IATA (thanks Zach Lipton for the link) a typical passenger on average creates 1.43kg of waste, of which 40% is food and plastic waste (~0.6kg). This is averaged across all flights, so presumably long haul flights will have more waste, and potentially a larger proportion of that will be food and plastic waste.
So where does this waste come from, and what can we do?

Snacks and miscellaneous

For small service items, like drinks, peanuts etc. the simple option is to refuse. Peanuts will keep, and so will the cups, cans, bottles etc. etc. to be passed onto the next person. (To counter the anticipated "but it’s going to be used anyway", the overall demand will be reduced, hence the airline won’t purchase as much).
For other things that might come in plastic, again you can just refuse to use them and leave them as they are. Be aware though that sometimes blankets can be wrapped in plastic, so if you think you’ll need one, you might have to wear an extra layer just in case.

In-flight meals

For things like the in-flight meals, it becomes a bit trickier.
If you haven’t planned ahead, then the moment the tray lands on your table is probably too late to do much about it. In fact, according to Dan, your trays are packed several hours before you even board the flight. What happens to the waste afterwards is in the hands of the contractor, and if you don’t use something from your tray, there is a strong likelihood it will be discarded anyway.

The link provided by Dmitry Grigoryev suggests that in some countries, anything you touch on international flights will be incinerated for biosecurity reasons (although there is a push to temper these laws).
However, if you aren’t landing in one of these countries, or your flight is domestic, some of your plastic might be reused.
To quote this news report from 2010: "Air New Zealand says its plastic utensils, including cutlery, trays, bowls, cups and plates, are sorted by staff for cleaning and reuse following international flights", although it doesn’t specify if this is light plastic or not.
It also mentions that plastic cutlery on some airlines is actually being sterilised and reused upto 10 times, and potentially as many as 30 times. Using it and returning it to the staff could be a reasonable option if you know the airline does this.

If you are unaware of how the airline handles used plastic cutlery, or you know they definitely throw it out, then the moment you receive the tray you should assume everything goes to waste. In fact, if you don’t say anything before your flight, then there is probably already a meal set aside for you, on a tray, ready to go.

Plan ahead

As noted by averell, budget airlines are often forgoing the traditional in-flight service, in favour of a "buy in advance" model (to coin a phrase). Choosing these airlines will allow you to virtually eliminate your in-flight waste by eating a sufficient meal before boarding, or preparing a light meal to take on board. If you are flying a full cost airline, then you usually have an option to specify your meal (people with dietary requirements will be well aware of this). In some cases you can request "no meal" and do the same as above.

If you take this route, then weight minimisation is something you should also consider, but you should be accounting for the upper limit of your CO2 emissions in your offset/budget anyway.

What impact will this have?

In exact terms, minimising your in-flight waste might impact your own personal environmental cost, but on the scale of the flight you take, the impact will be tiny. But this is true of almost any action a person takes in any sphere. The value of this comes from the message sent to the airlines about what their customers care about. As Harper points out, this is an issue with the entire system and requires ingenuity from a top level. SSight3 suggests documenting good practices you observe and passing them on to other airlines. But there is a lot going on behind the scenes that regular passengers simply won’t see.
Perhaps the best option is making sure airlines know that in-flight waste is a concern but telling them directly, perhaps through customer complaint channels. Again, the biggest environmental impact you can have is from not taking the flight in the first place.

I recognize there are many answers already, and some very good ones:

It is true that the CO2 is also waste, and that it dwarves everything else connected to it. It is also true that you cannot always avoid flying, and in this case it is also good to minimize the impact.

I’d like to focus on what you can do if you have to take the flight. I’ll use Europe/New York flights as an example.

Note that I didn’t look at plastic waste specifically, but it is safe to assume that burning 1 kg of plastic has roughly the same environmental impact than burning 1 kg of jet fuel – they are both made from crude oil.

You cannot reasonably avoid plastic waste for meals on regular flights

It is virtually impossible to commercially provide in-flight meals without single-use plastic; and there is no airline that does. Any attempt to “fix” rather makes things worse:

If the airline has complimentary meals, they’ll stock at least one meal per passenger. If you don’t take it, it’ll be thrown away, plastic included. Bringing your own meal just adds food waste to the equation. (On budget airlines it does have an effect to not buy the meal, see below).

You could opt for business class, where they have reusable dishes, but you’ll multiply your CO2 footprint that way.

Optimize your flights

The best way to reduce your footprint is to avoid as many flights as possible. For business, you may be able to combine trips – e.g. going from New York to London, taking the train to Paris and flying back from there.

You may also be able to travel less often, but stay longer each time.

Savings for taking one trip instead of two: Around 1900 kg (50%) per one-way trip.

Use new plane, go budget, go economy

These are probably the things that help most. Budget airlines will usually use new, fuel-efficient planes and pack a lot of people. Meals aren’t included; they’ll only pack as much as they expect to sell (meaning that not taking the meal actually has an effect and you can avoid the plastic waste as well). Many budget airlines don’t offer in-flight entertainment systems which saves additional weight.

All of this helps:

If you go to New York from London, Norwegian Air Shuttle uses new 787 planes, while British Airlines uses mostly 777s or 747s. And Norwegian fits 350 passengers into a 787, while BA fits only 215.

A “high-density” 787 flight will generate around 830 kg of CO2. A regular flight on a 747 will generate around 1200 kg; one on a 777 still 1060 kg. (A business class flight on a 777 will cause over 1900 kg).

Savings: Up to 370 kg (19%)

Use a different mode of transport, even for parts of the trip

If you’re in Berlin, and have a day to spare, you could get the train to London (41 kg) instead of the feeder flight (177 kg). Or take a train to Frankfurt (22 kg) instead of a feeder flight (130 kg) and a direct flight from there.

Savings: 136 kg (0,7%) or 108 kg (0,6%)

Weight matters, but not as much as you think

Using the values from other answers, an additional bag on the NY-London route would be around 14 kg of CO2 (23 kg * 0,2 kg of fuel per 6000km = 4,6 kg of fuel, times 3). This isn’t nothing, but it is isn’t that much either.

Also, buying new items at your destination may be even worse: Using the formula above, transporting a 0,2 kg shirt one way will cause about 0,1 kg of CO2 to be emitted, but producing a new shirt will generate about 3 kg of CO2.

Savings for one less bag: 14 kg (0,07%)

Compensate your flight

There are various offers to “compensate” your flight through a donation. While this doesn’t undo the CO2 emissions, they’ll use the money on projects to mitigate your impact to a certain degree.

How much this helps is up for debate – it is certainly better to avoid a flight in the first place, if possible. However, if you cannot avoid a flight you may give this a try.

Flight is waste, and extra weight matters

Coarse rule of thumb, by deciding to fly, total fuel weight divided by total pax weight is attributable to you*. That’s a lot – it’s your own weight in fuel, and probably quite a bit more.

As far as possessions, this isn’t a train. Every gram of additional weight adds to induced drag which adds to fuel burn. This isn’t quite as bad, but still, on a 10,000km flight, every 1 gram of possessions puts roughly 1 gram of CO2 into the air.**

If you bring a 20g metal fork, you just addded 20g of CO2 into the atmosphere. That’s worse than a 3g plastic fork and 3g of CO2 (the fork is also made of petroleum). And they won’t know not to load a packaged meal for you, so they’ll load it on the airplane anyway; that means the weight of your own meal is added to the fuel burn that will happen in any case.

Burning petroleum is worse than making packaging out of it and burying it in a landfill. Landfill is a kind of carbon capture, though certainly not a preferable one. When you landfill a petroleum product, it’s a wash – it doesn’t convert to CO2 since it doesn’t burn. So that 3g fork makes 3g of CO2 if it’s buried, or 12g if it’s incinerated.

Also, refusing the meal is unlikely to result in any environmental savings. The logic is “they’ll save it and serve it to someone else on a future flight” – not likely if it’s heated/prepared, since it can’t be heated twice (based on how aircraft galleys work; they’re ovens not microwaves). Anyway, due to biocontrols it’ll probably be destroyed at the destination.

Mind you, it also had to be grown in the first place, so you have all the CO2 load of factory farming to boot.

So you’re actually doing worse by bringing your own food. (unless that’s dietarily important; yet another factor to balance.)

If you do bring your own food, focus on lightweight packaging even if it is throwaway. My “go-to” is Ziploc bags. Better to make 1g of CO2 for a 1g ziploc bag, than 100g CO2 for 100g of quality tupperware. Twice since I assume you’ll bring it back.

Look at it a different way

Keep in mind, Big Minds are already working on the environmental-waste-of-aviation problem. It’s not as simple as “Disposable packaging bad”, and it’s a mistake to think “nothing is being done”. It’s more complicated and subtle than that, especially when you start thinking of the damage of poor biocontrols, e.g. disease, invasive species, that kind of thing.

Speaking of that: Luggage. Again, since your “stuff” takes fuel burn, make your luggage as light as possible. Don’t bring consumables that are readily available at your destination. That’s another “balance of priorities”: re-buy something you already own, or spend 3x its weight in CO2 bringing it with you.

Speaking of that: You. Every pound of you that you leave at the gym is a pound of fuel not burned. Also hit the airport bathroom before the flight; waste is not ejected; it is stored in holding tanks.

Distance matters

Because of the “tyranny of the rocket equation” – airplanes get better fuel economy as their tanks empty out. Their fuel economy is worst at takeoff when they are heavy with fuel – in fact they are unable to climb to the most efficient high altitudes until they burn off some fuel. Longer flights have worse fuel economy than shorter ones. And yes, it matters, because they don’t “top up”, they depart with only the fuel they need.

The gory math of whether “muliple short hops” is better (taxi, takeoff and climb-out costs a lot of fuel also, so maybe not), or whether straight-thru flight is better or worse than electric HSR – is too much for this answer, but it’s a place to look.

A sidebar: Waste-stream recycling is a thing

Recycling from intercontinental jetliners is weird, because of biocontrols. But to speak of recycling generally — Recycling efforts are already made on waste streams. Metals are separated by magnets or eddy currents; and laborers pick out cardboard, sacks of newspapers and grabbable plastics. Many cities have found it’s cheaper to have one waste stream and have machines/laborers separate recycling, than to have citizens have multiple waste cans.

Cities keep the blue bins due to citizen pressure/guilt, but it still needs to be picked through because people put lots of stupid stuff in the recycle bins. The upshot is, the “blue bins” are often “recycling theater”, and actual recycling occurs farther down the waste stream. Likely so if your destination is an eco-minded place like western Europe or the usual coastal ports of entry in the US. So the recycling may be happening even if it is not apparent!

* You decided to fly, and that makes you responsible for your share of the airplane’s weight, the fuel, and the fuel to fly the fuel. Just to throw a number out: An A380 carries 81,890 gallons of fuel. Say it’s super-tight all-economy, seating 819 passengers (makes the math easy). Each passenger burns 100 gallons of fuel. Fuel weight 6.8 lbs/gal. so 680 pounds of gas. That’s worst-case, but on a rather efficient bird. So I am very comfortable with “your own weight in fuel”. However since possessions and bodymass are incremental, we look at those differently.

** Based on a comment that every extra kg of payload requires 0.2l of fuel (nearly 0.2kg) for a 6000km flight, and 1g of fuel makes 3g of CO2, so 0.6kg. I am stretching that to a 10,000km flight and it rounds to 1kg/1kg. Mind you Jet A is about 15% lighter than 1kg/1l, except the tyranny of the rocket equation makes the fuel burn probably 15% worse, so I’m calling that a wash.

Bring your own food to the plane 🙂 This is the food I prepared for my February flight to India 🙂 It’s all vegan and raw, very rich and tasty. Avocado, brocoli, pickled cabbage, paprika, olive oil, soya sauce, lemon. Maybe I put some tahini as well. I made it very quickly, just 10 minutes before leaving. I can’t stand the plane food – it’s artificial, super processed food with lot of sugar and chemical preservatives. On the contrary, food like this one is quick, fresh, full of enzymes. It will make you feel satiated and fresh 🙂

I use good plastic box and my own spoon. I also take my 1L water bottle with me, and I just get it re-filled.

I also bring my own headphones and my own blanket from Cashmere 🙂 (these are the items that usualy go with plastic in the plane).

This way I literally waste zero plastic on the plane 🙂

Laudable as minimising waste on a flight might be, taking the flight create orders of magnitude more environmental damage than the little bit you’re trying to mitigate. Greenwash what you do in the cabin if it makes you feel better, but please don’t fool yourself that you’re doing anything remotely useful by it. You could produce a many decade-fold better result by not taking the trip.

In case of international flights, the amount of waste you will generate is pretty much defined the moment you buy the ticket. If the destination of your flight has strict biosecurity regulations (such as Australia, New Zealand, Europe and the US), everything you even may have touched will be destroyed: “literally everything — even a can of coke that has not been opened”.

So if you want to reduce the amount of waste generated by your travel, you should research airline eco-friendliness upfront, since your behavior during the flight will have little impact on waste production, barring deliberately wasteful behavior.

Throwing your waste into the bin/bag offered by the flight attendant is not mandatory unless the destination country has a strict biosecurity policy. If you care to that level about selective waste disposal the best thing you can do is to use on of the wrapping bags (or have one prepared yourself earlier), separate recyclable items and packaging from the general waste, put those into your bag and pick it with you out of the plane. Once on the airport you should find selective waste bins where you can dispose your recyclable waste accordingly.

If you’re not sure if this is allowed on a specific flight, consult a flight attendant.

BTW you inspired me to do the same on my future flights.

The easiest option is to politely refuse the in-flight service and to bring your own food and drink, with spoon and fork if required.

I remember some companies advertising that they recycle their waste, but information about that is hard to get. You could mail them and ask, and then choose to fly with those companies.

Note that by asking them you also send a signal.

My first reaction ss for plastic cutlery- order a business class ticket or better, they are usually serving food on real ceramic plates with steel cutlery and non plastic glasses.

But then I checked the Flight Carbon Emissions Calculator for a long haul flight and got that it uses around 0.5 ton per person per direction for economy class and double that for better classes- I suppose that due to the lower density of passengers on a plane

Comparing that to the average European co2 footprint which is somewhere around 7 tons per person per year, it means that your flight wastes 52 days of carbon footprint of an average European, and on business you will waste 104 days !

The only way would be to refuse the food package, as they always come prepacked with the cutlery and other items you may not want from the packaging facility.

SOME airlines may still offer reusable cutlery that’s washed and used again on another flight, most no longer do because of the higher weight, cost of ferrying the stuff around the world, cost of separating and cleaning it, etc. etc. (and the extra pollution caused by all that, which is often overlooked by those who’re opposed to disposables).

Anecdotal, but related, to that last: my father was a financial consultant to the dairy industry. In the 1980s when cartons first became available for dairy products there was concern about the increased amount of waste that’d produce. He ordered a university to do an end to end study of the total environmental impact (including production, transport, disposal, and cleaning of everything involved) and the study concluded that the cartons would overall give a slightly smaller environmental impact than glass bottles, using the then-available historical data on the number of times those bottles could be reused. The financial cost of cartons also was going to be quite a bit lower because of the cost of returning bottles to the factories for cleaning and refilling, which is higher than the cost of shipping in new cartons. This was unexpected, but helped convince the industry that cartons were the way to go.

A similar study for cutlery for airlines would quite possibly show a similar result (though probably a smaller difference as the weight difference between plastic and aluminium cutlery is rather small).

I doubt you’d be able to take steel cutlery into the aircraft cabin.

Food waste, including packaging, can be subject to biosecurity restrictions at the destination airport, and is often therefore just bagged up and incinerated. (Source)

Long-haul flights have impacts on the environment beyond the issue of cabin waste.
At the risk of sounding flippant, if you’re really concerned about the impact of your long-haul flight on the environment, don’t take the flight!