When #ChoosingARoommate, it's important to find someone who is compatible with your habits and philosophy. Otherwise you run the risk of turning your #apartment into a war zone. Apartment Therapy has a lot of great discussion topics for meeting potential roommates.
Make sure you choose someone with a similar schedule. If you like to stay up late, you don't want to live with someone who turns in at 9 and requires that the apartment be absolutely silent.
Another major bone of contention between roommates is housekeeping style. Make sure you are both on the same page about what constitutes clean and that you can split the chores in a way that makes both parties happy. Or invest in a housecleaning service.
Be sure that your potential roomie can pay their half of the rent. If they don't have a steady source of income or bounce around between jobs, that's a red flag that you might be left holding the rent check bag repeatedly.
Other important areas to discuss with potential housemates are decorating style, how often they have guests, and pets. Make sure you are on the same page about these things or can make mutually agreeable rules regarding them.
Finally, before you sign the lease, have a cup of coffee or a beer with your potential roommate and be sure they are someone you get along with. If you find yourself watching the clock so you can leave, that's probably not the person you want to come home to at the end of the day.