Perspiration is a normal physiologic response to the environment on an airplane.

Some factors, such as temperature and humidity, are obvious.

There is one that is apparently less so. Airplane cabins are simply not the environment most of our bodies are used to, no matter how frequently we fly. At cruising altitudes, cabins are pressurised at pressures equivalent to about 1800-2400 m (6000-8000 ft) above sea level.

Most of our bodies are not used to that. In response to the low barometric pressure, our bodies go into a mild state of hypoxia (lower oxygen concentration in the blood). This in turn will activate our sympathetic nervous system – that is, the fight-or-flight (haha) response. For more details see The autonomic nervous system at high altitude and Physiological Adaptation of the Cardiovascular System to High Altitude.

A consequence of increased sympathetic activation is the secretion of sweat from our sweat glands.

Your sympathetic nervous system may be further activated by stress, anxiety, or a number of other factors.

So there’s really nothing you can do to prevent your body from perspiring when you travel on a plane. Just let your body go about its usual sweaty business, and prepare for the repercussions.

Try wearing flip flops. If I wear regular shoes my feet get sweaty – and thus I feel more sweaty and unclean on long flights.

This works for me and I always do this (I fly 10-20 times per year).

I also couple this with comfortable lightweight clothes – t-shirt or lightweight casual shirt & shorts.

I have the same problem when I travel by air. My experience is that airplane seats don’t breathe like other fabrics/seats due to their secondary use as floatation devices (I’ve had similar problems with sitting in leather chairs). One of the best remedies I’ve found to at least remediate the discomfort is to use talcum powder prior to boarding, and reapply during long flights.

Another potential cause is that the total time sitting during travel can be quite high. I find that I have less sweat issues when I refrain from sitting in the boarding area prior to boarding.

To add some conjecture: If someone stores luggage under your seat, I imagine the airflow is cut down. Heat dissipation is likely the main issue here.

You did not mention whether you use the overhead air inlet (ventilation). It is there on every plane for your comfort. I open my inlet as soon as I get on the plane, and enjoy the nice and cool stream of air. It completely solves the whole sweating issue for me.
(Sorry not enough rep to comment)

All other conditions being the same, air travel has two physical aspects different from other modes of travel.

1. The air in the cabin is drier. This is why you are encouraged to drink water during your flight. Why would this promote sweating? The moisture in your skin would evaporate and your body is trying to compensate.

2. The pressure in the cabin might be lower than the atmosphere at ground level, which would increase the evaporation rate on your exposed skin.

I also tend to feel sweaty during and after long flights. I’m not sure if there is any hard research showing that it is or is not more common. I would personally guess that it is, but I don’t have any evidence to back it up.

However, it’s certainly possible that certain people do sweat more. You say it’s the case for you, and I feel that it is for me personally as well. Why is this the case for some people?

Long time in one position against seat

You’re typically sitting in the same position for many hours. This means that there is little or no air circulation between your body and the seat. It also depends on the quality of the seat, but it’s quite common for it to get clammy due to your backside being pressed against the seat for long periods of time.

Long time, no shower

Canton to Sydney, which is quite a long trip.

In this example, you may be traveling for 35 hours or more. In that case, there is a simple answer: You’re going for quite a long time without showering. (It’s, of course, possible to shower during the layover or even on the plane, but most people don’t.)

Normally, most people shower/wash themselves at least once a day. Some people do it twice a day. There are good reasons for that. Sweat, salts, etc. accumulate on the body, causing various uncomfortable sensations for oneself as well as others. Long air journeys can take much longer than that, sometimes causing you to not shower for > 50 hours. Perhaps, if you went that long without a shower unrelated to flying, you would notice some buildup of sweat too.

Airsickness

This may not be something you (OP) personally suffer from, but many travelers do. Cold sweating is one of the symptoms of airsickness.

Stress

I don’t know if this applies to you, but a lot of people can get slightly stressed while going on long journeys. They have to pack everything, remember everything, perhaps get up in the middle of the night, have their documents in order, keep track of their passport and money, be on time for everything, pass security, customs, immigration, guard against theft, wait in lines, experience lack of sleep, etc. Most people (including me, certainly), can feel mildly stressed due to all this. It is a well known fact that stress increases perspiration.

Lack of sleep

Even if you are a seasoned traveler who don’t feel stressed at all, and even if you’ve managed to shower during your journey, it’s likely that your journey interferes with your sleeping patterns. You might get some sleep during your journey, but it’s not going to be the same quality that you would normally get. Lack of sleep may possibly cause increased perspiration. Your body is tired, and has to work harder to perform its normal functions.

Airplane air

Here’s a final point that I expect to be (and welcome being) challenged on, but I’m including it anyway: The air in airplane cabins is extremely dry. This may cause the body to compensate by sweating, etc. Of course, water also evaporates much faster in the dry air. However, only the water evaporates. The salts and other components of your sweat all remain on your skin/clothes. This may increase the feeling of being sweaty. (Again, I do feel that this last point is somewhat speculative, so I’m including it more as a hypothesis than a fact.)

Prevention

Shower right before departing. If possible, shower during layovers. As @pnuts said, wear antiperspirant. Try to relax as much as possible during your journey, and try not to be in a hurry for or very worried about anything. Wear clothing that breathes well. Get up from your seat regularly and stretch a little bit. Bring extra sets of underwear/shirts/other clothing and change in the bathroom when you feel sweaty. You may even bring body wipes to clean your skin and reapply antiperspirant on clean skin. Do whatever you can to get enough sleep. If you can afford it, get business class which is much easier to sleep in. If you’re rich enough, and it’s available for your route, consider flight tickets that give you access to an in-flight shower.

Personally, I don’t bother with all of this (although I certainly do some of it). I’m just trying to answer the question on how to counteract excessive sweating, though. To which lengths you may want to go would depend on how important this is to you personally.