Portrait composition is the process of being able to blend two stand alone images into one convincing image. A while ago, I had a client who wanted to have some studio pictures taken for his music CD cover. I took two shots, close up and full body for every pose I directed. Here are some of those pictures and how I was able to use his portrait shots and composite them with a background.
I used Photoshop CS5 refine edge brush to carve out the subject from a white backdrop I placed behind him. Then I blurred the background a bit to create a small depth of field between the subject. I finished off by placing a Curves adjustment to make the colors blend with the entire composition. I think I did pretty good.