Here are a couple of still pics of the way we train shoulders. We stand on one foot on the bosu ball, forcing our core to stabilize while a weight is pulling it around. The weights, of course, are dumbbells, 10lbs or so, and as you do your shoulder exercises (in good form), the weight changes your balance point.
Basically, it is like recovering from a fall all the time. As the weight moves, you are constantly adjusting for it, trying to come back into balance.
Start these standing on the ball on 2 feet, and as you get comfortable, do a set with your leg in front, one with your leg out to the side, and one with your leg behind. You can also do them with your leg pulled up under you in a "flexion" posture.
Make sure your standing leg has a slight bend in it, and that you are using good form for the shoulder exercises, and good skiing form on your standing leg (ie, not in the back seat or other 'off' position.)
Shoulder exercises with dumbbells:
Cable Pulls for Rear Delt Raises
There are LOTS of others, but these are some easy ones to do o the Bosu ball. Some tips for better success: keep your shoulders level, and your wrists stacked. Keep your free foot out and steady using your abs for added stability, for the entire set.
Just like any normal shoulder exercise, do 3 sets of 12, alternating feet and foot position with each set. (For mass building, higher weight, fewer reps, go to failure and then go down 2 lbs, continue this until you have no weight and go to failure. For sleek, long muscles, do higher reps on lighter weight.)