Wave your bills at the bartender
It's like waving little presidential flashcards that read: "Hey! I'm a dick!"
Not tip your bartender well
Like professional elephant mohels, bartenders rely on big tips to make a living. Obviously, the size of the tip fluctuates to accommodate variables -- like how complicated the drinks are, how busy the bar is, how sexy the bartender looks, and how much money you don't have -- but even if you order a simple pint of Midori on the rocks, at least drop $2. Preferably in a $2 bill, because they are just neat.
I have a friend, I don't want to blow up his spot, so let's call him "Rob Alexson." I saw him throw up on a chair in a bar once. It was decidedly uncool. At least make it to the bathroom and try to up your chucks in a civil manner.