Before you do any rotational exercises, brace your abs forcefully—as if you’re about to be punched in the gut—and hold them that way as
you do the movement. This limits your range of motion and helps to keep
you from rotating excessively at your lower spine.
Abs: Mountain Climber with Hands on Swiss Ball
The benefit: It’s one of the simplest yet most effective ways to tighten your tummy. In fact, you’ll barely have to move a muscle.