I think you mean compositing (building an image from several parts) not composition (the plan, placement, or arrangement of elements or ingredients). When compositing you will make a composition, but not all compositions are composited. HTH.
I think the most important part of realistic compositing, beyond the basic skills of masking, selecting elements, and painting shadows, is having a critical eye and being able to figure out why something doesn't look realistic yet. Of course it helps to control the photo shoot so you can make sure the light sources are all consistent!