I like the advice given so far, however, I would just like to add this bit.

Traditionally, to prevent injury or rehabilitate the rotator-cuff shoulder muscles, isolated movements are recommended. However, the results of one recent study showed that multi-joint movements appear to be just as effective at strengthening these muscles as well as preventing a reduction in functional strength—one important benefit that is not obtained from isolation exercises.

This study compared the effects of isolated and multi-joint exercises on Wikipedia reference-linkrotator cuff strength. Thirty-nine males participated in this 6-week study. After initial isokinetic testing of internal and external rotation strength, the subjects were divided into 3 groups. The isolated-exercise group performed internal and external dumbbell rotation exercises. The multi-joint exercise group performed pull-ups, overhead presses, reverse pull-ups, and push-ups. A control group performed no exercises.

Both experimental groups showed an increase in rotator cuff strength. However, the multi-joint exercise group experienced significantly greater improvements in each arm.

The authors of the study do not recommend replacing isolated movements with complex movements to strengthen injured rotator cuff muscles. Instead, they recommend a combination of both methods.

Rehabilitation should begin with isolated movements to better stimulate the weak rotator cuff muscles, such as that mentioned below. This will help to increase local stability.
After basic strength has been restored, a switch to more complex, multi-joint exercises will provide greater synergistic recruitment of these muscle fibers that result in better improvements in functional strength. A progressive overload approach should always be incorporated to speed the recovery process.

Ref: Journal of Strength and Conditioning Research 18;1:144 – 148, 2004.

The key is to build up the local stability initially, and making sure that you arm has full available ROM. Thus, once local stability is gained, further improvements can be made by globally stabilizing and strengthening the supporting muscle groups of the shoulder.