Benefits: It's one of the simplest, yet most effective ways to tighten your tummy. In fact, you'll barely have to move a muscle.
How to do it: Assume a push-up position with your arms completely straight, but place your hands on a Swiss ball instead of the floor. Your body should form a straight line from your head to your ankles. Tighten your core and hold it that way for the duration of the exercise. Lift one foot off the floor and slowly raise your knee as close to your chest as you can without changing your lower-back posture. Then repeat with your other leg. Alternate back and forth for 30 seconds. If that's too hard, place your hands on the floor or a bench.