Reach over the bar to steal drinks
This is the one and only time a reach-around is not courteous.
Hose the bathroom down
Even if you are a man in the shittiest dive bar bathroom in the world, just remember: someone's sweet little grandma may come in later and need to tinkle. I know mine probably would. Aim to please, as they say.
Stay in the bathroom for longer than four minutes
The bar bathroom is a vessel for many (fun and/or necessary) things that often can't be done in the light of even the darkest, dingiest bar. Whatever you need to do can be done in four minutes tops. Trust me. I've done it. And by "it," I mean everything. And by "everything," I mean nothing, obviously.