Strengthening your core muscles is a great way to increase flexibility and muscle tone in your exercise routine. Adding a stabilization ball to your exercise arsenal is a terrific way to enhance the development of your core muscles.
Use your stabilization ball when you're doing sit-ups and push-ups. The ball can be used to add more challenge to Pilates and Yoga stretches.
When you've completed your exercise routine, you can also sit on your stabilization ball like a chair. Though it may seem like you're not doing anything while you are sitting, you're actually working your core muscles as you continually balance yourself on the stabilization ball.
"If you don’t have a ball of your own most gyms now have them available for use. When in doubt as one of the trainers at the gym to show you a few moves or inspect what those around you are up to. The ball can be used in so many ways and there are very few exercises that can’t be made to be more intense through the use of the ball."
http://astronutrition.com/blog/ball_exercises_core_strength