Do: You can do these on your toes or knees, or try them standing with your hands on the wall. Your arms should be straight but not locked, abs tight, back in a straight line. Lower until your arms are bent 90 degrees, then push back up.
Don't: Your midsection isn't the only part of your body that wants to sag; you may also notice your chest sinking toward the ground as your shoulders hunch up, says Jessica Matthews, an exercise physiologist for the American Council on Exercise. To improve shoulder stability, push through the heel of the palms as well as the outer surface of the hand.