Just my observations. Applies to both men and women.
1. You're anything but kind to those employed in the service industry. Cashiers, waiters, administrative assistants, tech support. If they mess up a request of yours, and you treat them like they're the dumbest creatures alive for it, then you're a douche. You're also illogical -- why yell at a waitress because your steak is the wrong temperature? You think she cooked it?
2. You deliberately display your underwear as part of your douchey outfit. It is not cool, it does not look good, and all it does is make you a big wedgie target. I also find that people who like this look on others are douches. Or terribly insecure.
3. You're unnecessarily rude to the opposite sex (or same sex, whatever works) when they try to talk to you. It probably took a lot of nerve for someone to approach you and ask if they can buy you a drink. Appropriate answer, other than "thank you, I'd love that": "No thank you" with a kind smile. Inappropriate answer: any variation of "haha, as if." Douche.
3 (a). Caveat: If you are friendly and say "No, but thank you," and they persist -- e.g., "Well, I don't see a boyfriend anywhere," or "What, you think you're better than me?" or anything else that implies you're just playing hard to get, then that person is a douche. You may have to resort to being a douche just to get them to go away. Some folks don't understand when people are direct, but whatever, communication is weird.
4. Turning the bass up so loud in traffic that the panels on your car vibrate. No one is interested in hearing your shitty music, and now everyone has to. Plus, for some reason it makes people like me sick to their stomach. Douche douche douche.
5. You drive a Hummer and are not military personnel. Extra douche points if you skid it out of every stoplight while exercising #4.
Okay. I feel better now.