Try this great twisting exercise to help firm up your mid section. Don’t forget to conract your glutes (butt) the whole time you are doing this exercise.
Set Up: Sit on an exercise ball with your feet shoulder-width apart while holding a medicine ball with both hands. Slowly walk your feet forward, leaning back on the exercise ball until your head and upper back are fully supported. Hold the medicine ball directly over your chest with your arms extended, but not locked, and lift your hips up until they are in line with your shoulders and knees (A).
Action: Keeping your body still from the waist down and your arms straight, lower the medicine ball towards your right side. Hold for a second (B); return to the start position and repeat on the other side to complete one repetition. Do as many as you can – if you’re a beginner, aim for 5 on each side.
Tip: Keep your eyes on the ball as you twist (source).