I regard the renders as merely a starting point to further, fine tuning of the image in Photoshop, so multi-pass rendering is essential ('Render Elements' in 3dsmax). Z-Depth is great for selecting background areas & applying atmospheric effects. Shadows & light can more easily be controlled if it's on a separate layer etc, I also made several Matte passes of various objects to make selections in Photoshop easier (a matte pass creates a mask of an object or selection of objects). I do a fair amount of colour correction & painting light rays & atmosphere and the specularity pass is good for picking out some highlights. The specular pass is also duplicated several times & set to screen, the occlusion pass is set to multiply. The opacity of all the layers is adjusted.
Working with passes is very specific to each image & what works for one may not work for another.
