Build functional core strength by doing side planks. According to Men's Fitness (UK) and Shape magazines this year, this is one of the BEST moves to shrink and tighten your mid-section to really get to your obliques and your quadratus lumborum, which is a small but very significant muscle of the low back that if weakened can cause chronic back pain. You may start on your elbow and build up your strength to do a full side plank. Hold as long as you can without dropping your hips. Repeat on the other side.