Working one arm at a time makes the core work more to stabilize your torso while you work your chest. Lie on your back with your heels about a foot away from your butt. Hold a dumbbell in your left hand, and bring both hands to the ceiling. Bend your left elbow, lowering your upper arm to floor. Exhale and straighten your elbow, pressing the weight back to the ceiling to complete a rep. Do 15 reps, then switch arms. Use a five- to 15-pound weight.