Free weights are the best way to strengthen the rotator cuff area. With a barbell (dumbbells are preferred since they help to isolate the shoulder area) you can add a lot of muscle and solidity to the area. Two exercises that really work the shoulder are flies and presses. To do a press, simply start with your dumbbells at shoulder level, elbows bent, weight right in by your ears. Then raise them up until your arms are extended over your head. Do three sets of 10, bring the weight up and down slowly to get full effect. With flies, you start with the dumbbells held at waist level and then bring them slowly out from the body until your arms are extended straight out. Do three sets of 10. You'll see strengthening in the rotator cuff area in days.
Resistance bands are a great, low-impact way to strengthen and build muscle. Plus, they are really easy to use. Once again, the main exercises would be presses and flies. To do a press, tie the band around a door knob, turn so you are facing away from the band and then pull the bands up to shoulder height and raise them from there until arms are extended. Do three sets of 10, taking your time to really work the muscles. With flies, you can keep the band tied to the door handle. Facing the band, pull either end out to the side slowly, focusing on the shoulder area. Do three sets of 10; more if you don't feel any strain.
Work and strengthen your rotator cuff and shoulder area by doing push ups. You can start by doing them from your knees and move up to doing them from the tips of your toes. Because the rotator cuff is sensitive you'll want to vary the amount of exertion you're putting on the area, thus going between the knees and the toes. Push ups are perfect for really focusing your effort and slowing it down to where you're working the shoulder muscles to their limits. Do as many as you can, take a rest and then do more.