How to check a large backpack?

What you are looking for is not called garbage bag although it certainly looks similar but a contractor’s bag. Your typical garbage bag will be 0.7-0.9 mil where a contractor’s bag will be 3 mil in other words 3-4 times as thick. Also, they are typically some 42 gallons meaning you will wrap the backpack in it several times further increasing the protectionHaving a bag like this folded in your backpack takes minimal space and weight.

Contractor bag — observer the 3 mil:

Ordinary trash bag (and this is thicker than most with 1.1mil):

One box with 50 bags will last forever if you only use it for flights.

Personal experience: We arranged our packs so there were no dangly bits to be damaged in the system. I would guess upwards of 30 flights this way, the total damage was one bit of the frame (note that this was long ago–external aluminum frames) was slightly bent but still serviceable. IIRC this required using some extra strapping materials.

Obviously there would be loose ends on straps but they were always wrapped around and tucked under other straps so they didn’t dangle.

The backpack I have has a pocket at the bottom with a cover that zips over all the straps sort of like what alanh mentions in his answer. That’ll only help you if you consider this important enough the next time you buy a backpack, though.

Apart from that, I’ve also got a combined rain cover and transit bag. Something like this:
http://www.gapyeartravelstore.com/highlander-backpack-bag-raincover.html

It’s basically something halfway between a duffel bag and a rain cover.

Not tried, but it strikes me you could use a baby stroller bag like this one. They’re meant for use at carry-on/gate check, but I reckon there’d be no problem checking them in.

They also fold down quite small so you’d not waste too much space while you’re backpacking.

If you buy a travelling backpack, i.e. a backpack that opens like a suitcase, normally they will have a pocket that you can zip the straps into, which keep them out of the way when checking in. Like this (courtesy from MEC’s website):

The other thing I’ve seen some bags come with is handy nylon+velcro organisers for straps. Kind of hard to describe, but here goes:

• Short length of strap-style nylon, about 4-6 inches long.
• Velcro on each end
• Place horizontally across the long dangly end of your strap after tightening all the way/clipping where possible
• Wrap the dangly end around it, rolling up into ball/cube/thing.
• Fasten velcro.

The bag I’ve seen them on is this: http://www.tactical-kit.co.uk/hazard-4-second-front-12937-p.asp, and they can be seen in the photo of the tan bag with the waist strap to the front, on the side of the waist strap.

Once, when checking an external frame backpack onto a Virgin Atlantic flight I was offered a giant, durable, resealable (great for security checks), clear plastic bag. It was similar to a clear trash bag but thicker and had the airline’s logo on it. It seemed like a good solution prevent any straps or hooks from getting caught in the baggage handling.

Since that flight I’ve found that most legacy airlines have these. I’ve often encountered check-in agents that didn’t know their airline had these, but in every case they asked their colleagues or manager and discovered they did. I’ve never been charged for one. I’ve gotten these in Europe, Asia and North America.

Here’s an example of a bag I caught a photo of coming off a Delta flight from Tokyo Haneda to Minneapolis:

You can also use the (paying) plastic wrapping services that are often found in the airport terminal. This will ensure that all straps, handles, etc do not protrude, and thus will not be caught in the tapis roulant. If TSA/Security or whoever needs to access your bag, they will cut through the wrapping and should then replace it with their own heavy-duty wrap/duct tape.

For more information see this question.

In terms of the advantage, any wrapping method you use must be heavy duty, or at least capable of withstanding rolling conveyor belts, collisions with other luggage, baggage handling roughness, tarmac, rain, etc. That’s why cooking plastic film might not do the trick, unless wrapped many times, and jpatokal mentions heavy duty garbage bag.

Checked bags may be opened during screening. So the plastic wrap would be removed.

You could get a duffel bag with a draw string and put your backpack in it. That would offer more protection then a plastic bag, but it would take some room in your backpack to carry around when your not using the duffel.

The TSA’s site has some tips.