One of my favorite shoulder exercises is a:
Modified Bent Over Row.
A great exercise for arm and shoulder definition and strength.
Pictured is an advanced move where I am one legged and balancing on a bosu ball.
Always start on the ground with both feet on the ground whenever trying a new exercise.
Work yourself up to a point where you can confidently add balance for an even more effective workout!
It is also awesome to move this into a tricep extension when you get to the top of the move.