Ive been using my Shot1 as a test to test out some render layers, i decided to render out a diffuse pass, a beauty pass and an ambient occlusion pass, i also rendered out the master layer, and then took it into Photoshop and had a play around with the multiply settings to see what happened. In all honesty i didnt really know what i was doing, however i managed to get an improvement on the original that i rendered out. The bottom picture in the list is the composite, you can see that it has a more natural highlight,which is especially noticeable on the tree in the foreground compared to the image above it which is the original without passes.
Here is a great tutorial i used to learn about render passes. Its in 2 parts, however this is part 1.