Take a small weighted ball (~2-4lb) and lay face down on a table, bench or physioball. Get your shoulder and elbow into a 90/90 position. Toss the ball up by externally rotating your shoulder. Catch the ball and then quickly throw it up again. Repeat this sequence for about 30 seconds. You should feel the muscles on the back of your shoulder blade doing all of the work. Avoid compensating by shrugging your shoulder or letting your elbow drop below shoulder height.