Ambient Occlusion is a nice effect which you can use (in theory) to create shadows/dirt masks for models.
Here is a nice tutorial for a normal 3d viewport render:
http://www.everflow.com/support/tutorials/AO/Everflow_AmbientOcclusion1.htmlUsing it with render to texture is a bit trickier. Here is a thread discussing several ways to do it, but all of them result in a black render for me. :/
http://forums.cgsociety.org/showthread.php?t=205378&highlight=ambient%2BocclusionIf it works for you, this is a great way to simply add some nice shadows to a skin.
Here is an example w/ the stairs:
This on top w/ multiply blend mode & ~ 50% opacity

Normal color render (ignore odd shadows on plane :p )

Combined =