Hey Si

Another thing I do as well as rendering the seppreate passes for the basics (colour, shaddows, specular) ill create new scenes, say with occlusion, reflection and then render the lot using a .bat file that way you can get maya to render sepperate scenes, layers, passes etc that you cant get from the basic layer controls, which when compping gives a lot of control. The downside it that you need the computing power to do it as it tie's up a Pc for a while depending on how complex the scene is.