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To make an ambient occlusion pass (or at least something close), I color everthing in the scene pure white with a material that has the diffuse, specularity, and ambient properties turned to zero, and the Occluded Ambient channel set to something like .9 . Render in best mode. You don't even need to have any lights on in the scene.
Not perfect, but it works nicely for darkening edges and depressions. I then multiply this pass on top of the color, adjusting levels for best look.
I also do occllusion passes with a white fast shader and several lights coming from every direction. Check out the Ambient Occlusion Light Setup. Each lights have shadows, only the depressed areas keep the shadow (because the light fills in everywhere else). I sometimes use both, or none...