I'm going to start this one with a quick question: What is the primary function of the rotator cuff?
If you answered external rotation (or internal rotation), you are wrong. Even if external (and internal) rotation is a function of the rotator cuff, it is not its main function.
The primary function of the rotator cuff muscles is to stabilize the humeral head in the glenoid fossa. Your rotator cuff needs to stabilize the humeral head to prevent it from moving in the joint capsule when you press, row, throw, etc.
Now let's consider the type of exercises we use to train the rotator cuff. We all typically use some kind of external or internal rotation based movement at the shoulder to train it, right? Now that we know that the primary function of the rotator cuff is one of stabilization, we might want to incorporate rotator cuff exercises that will train this function.
The most basic exercise for this purpose would be the supine dynamic stabilization with the help of a partner. The goal is to resist any random perturbation the partner is going to apply to your arm and try to prevent it from moving. (The perturbations the partner apply to your arm should be light pushes, they should be random to keep you from guessing which way he's going to go next and there should be at least a second between each pushes so your shoulder has time to stabilize completely).
There is a variety of progressions with this exercise alone just by getting your arm in a different position, changing your body position, or even by just closing your eyes (which will increase the proprioceptive challenge of the exercise since you can't anticipate the perturbations). Here's one variation in the half kneeling position with the arm abducted.
And finally, the 1-Arm Wall Stability Ball Hold, that we use quite a bit at Endeavor with a lot of our athletes. Kevin Neeld actually came up with this one. At first you're going to see the basic exercise, and then a progression we use by simply adding a partner perturbation to the ball or forearm.
For a better understanding of the rotator cuff and how to train it for optimal performance, I would highly recommend you check out Eric Cressey and Mike Reinold's new product: Optimal Shoulder Performance. I attended this seminar back in November, and I have to admit that the investment was worth every penny (I'm actually the big fat head in the red hoody we see constantly during the lecture part on the DVD!).