Often times brushing your teeth isn't enough to kill all of the bacteria in your mouth that create bad breath. Try using a mouthwash, e.g. Listerine, Crest, or Act.
If it's really persistent, and the mouthwash/more brushing/flossing don't help, then talk to your dentist. It could be the result of a dental or gastric condition... assuming that you don't eat cloves of garlic by the handful.