In yoga, it's important to work your whole core, which includes your lower back as well as your abdominal muscles. Overworking your abs alone. can create back strain, which nobody wants! Some great yoga poses that will really use your core muscles are Plank Pose, Side Plank Pose, Boat Pose, and any balancing pose, such as Tree, Eagle, or King Dancer. Check out the iSport guide, "Yoga for Your Core" for other ideas:

http://yoga.isport.com/yoga-guides/yoga-for-your-core/

Also, it's important to note that all the belly-toning exercises in the world won't make your abs flat if you have too much body fat. It's impossible to lose fat on single spots of your body, so make sure you're including fat-burning cardio along with your yoga practice, if weight-loss is your goal!