Most yoga moves require abdominal strength, which makes them really good for trimming down your muffin top. The following two moves are particularly good.
Fixed Firm Posture
This is good for the knees and spine but also for the abdomen.
Go down into a kneeling position with your bottom sat on your heels. Now lean back, placing your forearms on the floor and tilting your head backwards until you are nearly horizontal. Hold for 20 seconds.
This is good for stretching your body, as well as strengthening the abdomen.
Go into the kneeling position that you used above. Inhale and lift both of your arms up, bringing your palms together and stretching. Slowly exhale and pivot down from your waist until your fingers and forehead reach the floor. Hold for 20 seconds, then inhale and reverse the moves that you have just done.