What you need: An exercise mat or towel, and a pair of five- to eight-pound dumbbells.
Before you begin: Warm up for three minutes with a brisk walk or by stepping from side to side on the spot. Then, standing with your feet hip width apart and knees slightly bent, raise your arms out to your sides and gently squeeze your shoulder blades backwards toward each other. Isolate the movement in your mid-back, and avoid waving with your arms; your shoulders should move only an inch or so. Repeat for one minute.
How often: Begin with two sets of each of these three exercises; you can progress to three sets as you get stronger. Do this routine three times a week, on non-consecutive days, and reserve a few minutes afterwards for gently stretching the muscles that you have just exercised.