While standing with a shoulder width stance, take a medicine ball and press it overhead. Hold it overhead and bend laterally (side to side) while actively resisting the movement with your abdominals and obliques. Overhead Standing Side Bends are a very good exercise to build up strength and stability in your core.
Overhead Standing Side Bends with Medicine Ball - Start Position
Overhead Standing Side Bends with Medicine Ball - Finish Position
Overhead Standing Side Bends with Medicine Ball Exercise Video