When I fly, I dress for two things; comfort and safety.
Comfort - I tend to find the planes very cold, and will always wear long pants and a long sleeved shirt, usually a sweatshirt. If we're going to Disney, I'll make sure I have a short sleeved shirt on underneath, and may pack a pair of shorts to change into once we arrive at Orlando. I'll usually wear comfy jeans or dockers. My feet are always cold, so I always have on heavy socks, and full shoes (not sandals).
Safety - It's a terrible thing to think about, but you do need to consider safety when flying. I always make sure that I'm wearing natural fibers, not synthetics, since they are safer in a fire. I might wear a polar tech jacket over everything else (easier to wear than to pack anyway), but I tend not to wear it actually on the flight, and I'd like to think that I'd take it off if there were any problems on the plane itself.
Once my comfort and safety are taken care of, I try to be sure I don't look terrible; other than the jeans, I dress sort of business casual. I do think you get treated better if you're dressed nicely.
My two cents! Hope I didn't bum anyone out!
Jill Neill