A roommate ad should support your desire to find the perfect roommate and spell out the specific characteristics and attractive traits you want in a roommate. The more specific you are in what you want, the better result you will have.
Once the ad is active, people will begin calling you. Prepare several questions to ask them to get a sense of who they are and if they would be a good fit for you.
Make sure you have everything in writing. You want to make sure all your dealings with your roommate are in written format.