These are like sit-ups, only way worse (but in a good, ab-defining way). "This exercise is great because it really works your whole core," Ferraro said. "You hold a medicine ball at your chest. When you sit back reach behind your head with the ball, and feel that stretch. When you sit up, press the ball to the ceiling. This mimics the motion of a punch, and also helps tone the arms!" He recommended doing three sets of 10 to 15 reps.