Boots, speedos, Ray Bans and a cowboy hat.
Probably jeans and a sweatshirt would do. Don't wear anything too good because it will get covered in grease, oil, fuel etc. Don't dress up like Goose and Maverick to fly a Cessna as you'll look a d1ckhead (although I wouldn't begrudge anyone using a flying suit for genuine reasons). Don't wear synthetic fibres, again for safety reasons. Many people do it but I'm not a fan of too much exposed flesh (eg shorts and t-shirts) in aircraft - I've seen the difference even a thin layer can make in a flash fire. Dress comfortably and remember that it's a lot colder up there than down here. Have fun!