I think the fact that you're a little shaky shows that you are working your stabilization muscles. Stabilization muscles help to stabilize you, or keep you from being shaky. When you get stronger and your form improves, the shakiness will decrease significantly.
To answer your question more generally, the best way to strengthen those muscles is to do work on an uneven surface. For example, doing pushups on a bosu ball. The muscles you need to balance yourself as well as complete the move > completing the move by itself.