Prone Leg Lift
Core exercises such as this Prone Leg Lift are very important to make the back stronger. Swiss balls are great, but can be intimidating. You can do this prone one-leg extension with no equipment or add a pillow for comfort.
Lie on the floor flat on your stomach. Place your forehead on back of your hand. Lift the right leg as high up as possible towards the ceiling with the knee straight. Hold for 3 seconds. Repeat with other leg.