My shoulder warm up consists of warming up the
rotator cuffs. Once those are primed I don't even need to worry about he rest of my shoulder girdle.
Take a light dumbbell (5 pounds or so), upper arms parallel to the floor, elbows bent at 90 degrees (kind of like you are making a goal post with your arms). Then rotate the forearms forward while keeping the upper arms stationary. Great warm up! Weight can also be added (slowly, over time) to strengthen the rotator cuff muscles and prevent injuries.