Establish general rules to minimize conflicts. For example, set visitation hours and noise level restrictions so your roommate doesn't start a party when you're trying to study in the same room. Even if your apartment doesn't have a pet policy, you'll want to set specific rules concerning animal additions.
Dust accumulates. Dishes pile up. Eventually, the living space will require cleaning. Decide who will take care of the chores. Set a deadline to minimize procrastination. Be fair by dividing the tasks and alternating them weekly.
If you pay expenses, divide the responsibility. For example, you can take care of the gas and electricity bills while your roommate handles the Internet and cable bills. You don't have to pay the bill in its entirety, but take responsibility for collecting the money and getting it paid in time.
Don't barge into your roommate's room and borrow items without asking. Expect your roommate to respect similar boundaries. Even when it comes to groceries, don't borrow food without permission.
Even if you have few things in common, attempt to spend time together. Watch a movie in the living room, play a board game or just have a friendly chat. By taking shopping trips together, you can save on gas money and avoid overstocking the apartment with sharable items such as dishes or cleaning appliances.
If you feel your roommate has wronged you, don't stew in tense silence. Talk about it. Be polite and avoid raising your voice or piling on the blame. Recognize when you're making unreasonable requests or exaggerated complaints. If an argument ensues, work toward a compromise.