Seated External Rotation

Here is a good exercise for warming up the rotator cuff, especially before performing any type of pressing motion.  Start by sitting at a bench that has a support, like a preacher bench. The arms (humerus) rests on the support bench. The shoulder chest junction should be 90 degrees.  The elbow junction should be 90 degrees as well.  Holding the dumbbell, go up and down keeping the arm at 90 degrees.  You should do as many reps as you can until you feel a burn or fatigue in the rotator cuff.