When I first moved out I ended up rooming with two guys that it turned out were hard to live with. They were friends who I trusted, but their habits were less than desireable. In the end I moved out from there after about 4 months and got an apartment with a buddy who turned out to be great to live with.
So, main advice. If you don't smoke, don't live with a smoker. Even if he agrees to smoke outside, everything about him will smell like smoke and the smell will drag inside. Do the dishes, take out the trash, clean up after yourself. I'm not saying you should be the only one who does it, but don't be the guy who doesn't do it at all. Common areas are sacred, keep them clean! Roommates girlfriends are OFF LIMITS ... as are sisters and close female friends (unless cleared beforehand). Alarm clocks should always be turned off if you will not be around when they go off. Nothing sucks more than a roommate locking his room door and not turning off his alarm clock, but it's loud enough to wake you too.