One of the things I like to do is take something that exists in the real world and try to recreate it digitally. In this example, I extracted the helicopter on the right and replaced it with my helicopter. The key was matching the lighting, color, blur, and grain of the original shot. What is really great about this technique is having a reference for what things are supposed to look like. Then when you are working on a 100% CG shot you will have the proper chops… I mean skills to get it done. These kind of excercises will give you that real world experience needed to become a great compositor.