Dress with practicality in mind. The airport not a catwalk. Airports and airplanes are full of tired holidaymakers who have no desire to look their very best.
Mix and match. may be slightly worn bottoms and your favourite, more dressy top. Or vice versa. Your whole outfit doesn't have to be high fashion, but one piece could be.
Wear a comfy outfit and dressier accessories, like a fashionable carry on bag or nice jewellery.
Try to think about things like going through security. If you can avoid it, don't wear a belt as you'll have to take it off. Wear shoes that are easy to take off and put back on, not the ones that take ages to lace up.
Don't wear awkward things like high heels, as stepping onto the plane might be pretty tricky and you'll have less chance of escape if you have to get out in a hurry.
carrying things with offensive slogans or images isn't recommended. You might not get an easy time from other passengers if you're wearing a t-shirt with a racist joke or a picture of a middle finger. There will be thousands of different people with different beliefs, be respectful of that.
Wear a tracksuit and trainers if you don't want to use these tips. No byody will judge you for looking slightly sloppier than usual. They'll be too busy rushing around to know or care what other passengers are wearing!