The benefit: This exercise works your glutes and abs as hard as it works your chest and triceps. So you'll tighten your hips and core as you tone your upper body.
How to do it: Grab a dumbbell in your left hand and lie on your back on a flat bench. Hold the dumbbell over your chest with your arm straight. Your palm should be facing out, but turned slightly inward. Place your right hand on your abs [B]. Lower the dumbbell to the side of your chest. Pause, then press the weight back to the start. Do all your repetitions, then repeat on your right side.