I recently purchaeds a stability ball to UP the intensity of my AB workouts. I really enjoy the ball, and I recommend one to all of you. Here are my fav's, give them a try!
Roll and Curl
Begin in a full push up position with tops of your feet resting on the ball. Hands on the floor shoulder width apart. Keeping your abs and glutes tight bend your knees to roll the ball in toward your chest. Roll the ball back out then back in toward your left chest, then right chest. Repeat the sequence 12 times.
Use the ball to support you, feet hip width apart. Do 25 reps.
Ensure that your abs, glutes and hamstrings are tight. Hold the position for 30 seconds.
Oblique Crunches on the Ball
Balance yourself on the ball like the picture below. Tighten your abs, and lift off the ball using your side ab muscles. Ensure you don't twist and turn your body. Repeat 15 times per side.
Repeat the entire sequence 5 times (or as many times as you have time for). When you first start using a ball it can be a little frustrating, it does wobble around.