This one should be pretty simple, really. Think of someone that you have very strong, positive feelings toward. Got that person in mind? Good! Now consider how you treat that person. This is how you should try to treat everyone else on the planet. Even if they're being a douchebag to you, don't sacrifice the groove by dropping to their level. Respect them, respect their wishes, and let them live their life however they want to, even if it makes you mad as hell! You can agree to disagree without violating this mantra, as long as you do so with compassion, calm, and an acknowledgement that their life is just that: Theirs (not yours)!
So don't pick fights. Don't use language designed to hurt, belittle, or marginalize anyone. Don't insult someone's lifestyle. Don't try to force your ideas or life choices on anyone else. That's how you not be a douchebag.