Here's an example of how I approach some of my paintings - mainly the ones that I don't plan out.
Note: I don't like using a lot of layers - at most two or three at a time. I'll paint my base jazz first, then I'll just paint over that. If I'm not sure about how something is going to look, I paint it on a new layer. When I like it, I'll merge them all together (except for the background layer, that's always separate).
edit: sorry for the re-submitting craze...I can never get the size right, so it might be a lil' small....