Sit on a stability ball with your feet flat on the floor. Walk your feet forward, rolling your back onto the ball until your thighs and torso are parallel with the floor. With your hands behind your head, contract your abs to lift your torso off the ball, pulling the bottom of your ribcage down toward your hips. As you curl up, keep the ball stable and avoid using your lower body to push up. Return to starting position and repeat. This exercise is great for your upper abs.