Typically, to develop power in any muscle you must train those muscles using fast, explosive movements. An example exercise that can be used to help increase power in the shoulders would include an overhead medicine ball throw. Perform 2-3 sets of 8-10 repetitions. It will be important to note, however, that for one to be powerful, they must also be stable and strong. So one should first develop ample amounts of stability and strength before they begin power exercises. For example, you can begin by performing a ball dumbbell scaption as an exercise to improve stability through your shoulder complex. You can then progress to perform a strength specific shoulder exercise that allows you to lift heavier weight, like seated overhead dumbbell presses. Finally, once stability and strength have been developed, you can start performing your shoulder power exercises. This will ensure you're prepared to handle the stresses that come with these power exercises and you'll get the most out of them.