This is what I wear, honed from periods of extensive travel for work and a lesser amount of vacation travel. YMMV.
I always dress in and bring layers.
I bring a fleece jacket which usually lives in my personal item (tote) unless I need it, though not only is it useful for cold airplane air, but also to smunch up as a pillow. This jacket also comes in handy at destination, even a warm one, if there are for example cool evenings. (even departing Canada in the depths of winter, I can get away with: long sleeve top, fleece, and waterproof/windproof rain coat -- all of which individually are useful in a warm destination; the layering is great ! And those three together are enough even at -40 for the short duration one is outside between house and car and car and terminal -- if I drive to the airport I park in covered parking, which insulates some from the cold; otherwise we use a cab, which takes us right to/from the terminal; I also add scarf, hat and mitts for winter; and the mitts in the hat, rolled in the scarf can make a nice neck rest on the plane or at the hotel). (the outer rain/wind jacket gets put into the carry-on bag since it won't be needed once we are in the airport/in-flight).
I wear
loose fitting black fitness or yoga-style traveling pants. The ones I have are by Ex-Officio, but I don't think they make the style anymore. They are comfy, flexible and I have used them as my pants when traveling by plane for the last several years. Because they are black, made of substantive material, and drape well, they can also double as dressy pants or casual pants during the trip. I bring one pair and wear it on both the trip to and from. I usually wash it once I arrive at destination if I have the chance. They have an elastic waist, so no need for a belt.
I wear a loosely cut cotton or cotton-blend top; often a loose long-sleeve t-shirt, but sometimes a short sleeve polo shirt. It is a judgement based on the weather at destination. However, I have some nice lightweight long sleeve t-shirts which can make the transition from winter to summer prretty well, so they are my preference. This is also where having that fleece comes in handy
At home I always wear an undershirt under my tops. I do this when I travel as well. I like the Tees by Tina brand, and depending on the sleeve length of the outer top, will wear either the short sleeve or tank top style. It contributes to layering, and helps prevent overheating, moving moisture away from the human
I also like Ex-Officio's Give-N-Go women's tank top and men's short sleeve tee (which fits just fine on a female) for the same reasons, and the Ex Officio underwear tops do an even better job of moisture management and are designed for travel, so wash and dry easily if needed while traveling.
Comfy underwear. Sitting on an airplane at 35,000 feet for however many hours is NOT when you want to have uncomfortable underwear that rides up, chafes or does other undesirable things. Everyone will have their own preference in this department, but whatever you choose make sure it is comfortable !!
Bra. Again, comfort and proper support are important. Underwire bras will often trip the magnetometer alarm, which in my experience ends with a search by wand or sometimes a pat-down. I don;'t have a problem with this, but others might, so consider your bra choice carefully.
Comfy socks. Comfortable fitness/walking/running-type practical shoes - both for the practicality of moving around airports and thus walking quite a bit, and also because should the plane need to be evacuated, I want safe footwear on my feet that can stay on my feet (I have been on 5 planes that had to have emergency landings, one of which came rather too close to us having to evacuate for my comfort; therefore, I prefer to be prepared for the highly unlikely eventuality). These are the same shoes I will wear in the parks if a WDW vacation.
No belts, and the only jewlery is watch, fitness monitor bracelet, (both of which come off to clear airport security; I put them in a small zippered pocket in my personal item before we even enter the screening area, and put them back on either on the other side of screening or at the gate), MedicAlert bracelet (which never comes off, but it does sometimes set off the magnetometer alarm) and plain stud earrings (they also stay in and have yet to alarm on me). My hair clip is a Goody plastic clip, no metal. Car/house keys go into a specific pocket in my purse (so I know where to find them at the end of the trip; same pocket each time) once we arrive at the airport, so are not an issue for screening.
Hope this helps.
-SW