I always dress for comfort and practicality. For long-haul flights, I find jeans too heavy and restricting to sleep, so generally wear a smart/casual pair of trousers like Dockers which I find much more comfortable than jeans.

I always wear a micro-fibre shirt that does not need ironing, and prefer to have a pocket in the shirt to keep my passport and boarding pass while walking through the airport so I don't have to keep digging into my carry-on bag. On BA flights, I always wear a short-sleeve shirt since their cabins are always a few degrees warmer than I would prefer. Otherwise the sleeve length suits the destination weather conditions, and since I often change hemispheres, that often means changing season between source and destination.

I find my most confortable shoes for both walking and just wearing for long periods.

On long-haul flights I always carry two spare pairs of socks, two spare shirts and two spare pairs of underwear, and will grab a shower at every opportunity I can at intermediate airport lounges.