This exercise not only strengthens core muscles, but it teaches you to dissociate what your lower body is doing from your upper body. Hold on to an exercise ball or a stable base.

Stand in a shoulder width stance and bend slightly from your waist.
Stabilize your lower body and you rotate the ball with your hands and arms to one side.
Roll the ball over to the other side as you keep your lower body stable.
Repeat rotating ball side to side for 8 to 10 repetitions on each side.