Usually the guys who I think are douchebags don't dress a certain way, it's all about how they hold themselves and interact with other people. They give an air of "I'm all that, yeah, look at me" kind of thing. There is absolutely nothing wrong with a guy dressing well or liking to look good. My boyfriend wears polos, guys who wear button-up shirts and other things can be amazing people. But there are plenty of guys who wear jeans and a tshirt that come off as douchebags because of how they hold themselves. IMO :)
Douchebag: "An individual who has an over-inflated sense of self worth, compounded by a low level of intellegence, behaving ridiculously in front of colleagues with no sense of how moronic he appears."
Have you ever heard or seen the MTV show "Jersey Shore?" If not, I suggest you dress like the Sitch whom IMO is the friggin King of douchegabbery. Also, you may want to throw in a few fist pumps too for effect.