I thought I replied to this thread but it was probably another thread on the subject of the rotator cuff. Anyway, the L-fly is probably the most important exercise one could do for the cuff. Here is an explanation by Stuart McRobert (of Hardgainer magazine and Beyond Brawn), rather an excerpt from his excellent book (I recommend everyone purchase this) on weight-training technique.
"The L-fly is needed to reduce the strength imbalance between the external (weaker) and internal (stronger) rotator muscles of the shoulders. An excessive strength imbalance between these opposing muscles is a major contributing factor to shoulder problems.
"To distinguish between the external and the internal rotators of your shoulders, imagine that you are shaking someone's hand with your right hand. Keep your right arm bent at a right angle, and your elbow fixed at your side. Moving your right hand to the right is external rotation. Moving your right hand to the left is internal rotation.
"Small discs are especially critical in the L-fly, to ensure that progressive resistance is applied gradually.
"To do the lying L-fly, lie on your right side on a bench while placing your right hand on the floor for balance. With a small plate or very light dumbbell in your left hand, form a 90-degree angle at your left elbow. Then put your left elbow on your left oblique muscles (or hip, depending on your body structure). Lower the weight until your left forearm rests against your abs, then raise your left lower arm as far as possible. Always keep your left elbow against your side. Inhale on the descent, and exhale on the ascent. Finish the set, turn around, and then work your right side.
"Do the exercise slowly -- at least two seconds for the lifting phase and another two seconds for the lowering phase. And use a weight which permits you to perform at least 8 reps.
"Start with no more than 5 pounds for a man, or half of that for a woman. Add weight VERY SLOWLY. Depending on training experience, a man may need a year or more to build up to using just 10 pounds. This is quite a lot of weight for this movement; and 15 pounds may be enough for even a very strong man.
"Never train shoulder external rotation to failure. Keep the very last rep in you. Never get to the point where you need to raise your elbow or roll backwards a little."