Communicate openly and honestly. Your roommate can't read your mind, so if something's bugging you, bring it up. If you wait too long and let the problem escalate, it becomes more unlikely to be solved.
Know when to hold 'em and when to fold 'em. Pick your battles carefully. Sometimes it's easier to accept someone's irritating habits than to argue about them.
Keep a life outside your apartment. Most people choose to live with friends, and understandably so. But spending every waking hour with someone can put a heavy strain on the closest friendships. Try and maintain some separate friends or activities to give you a break.
Try to remember that you're friends. Set aside some time for all those fun things you used to do before you had to fret about leaky faucets and overdue rent checks.
Speaking of which, keep on top of those rent checks. Nothing causes more arguments than money. Solve that by putting up a whiteboard where you can write reminders about paying your bills.
