As others have said, keeping the grip hand as low as practical will help protect the shoulder. My paddles have tended to get progressively shorter over the years. The shoulder joint is most stable when the humeral head is low and back in the glenoid and least stable when it is high and forward. Unfortunately, I have found that effective cross forward strokes, essential in paddling small, solo canoes in whitewater, tend to be hard on the off-side shoulder, especially if done incorrectly. Concentrating on keeping the shoulder low and back will help some.