Grab a stirrup handle and put it on a lat pulldown machine. Facing the machine place the handle by your ear, either right or left. Bring your hand down to the opposite side of your body bending only at the waist and letting your abs bring the wieght down, not your arms. Come down enough to get a good squeeze. Perform same movement for opposite side. 10-12 reps for 3 sets. When that gets too light go up in wieght. That will do the trick.