Lie on your stomach on the floor, or balance on a physio ball, holding 3-pound dumbbells in each hand. Engage your back and lift the chest a little. Then, move arms up and out into a T position, release, move into a Y position, release, and then move them into an I, arms touching out straight above your head. This is a great one for the rear delt, which is an important posture muscle, Stokes notes. Most people are very weak here, so use a super-light weight for this one.