To render AO you need at least one light activated that can cast shadows. Don't forget to activate AOcclusion in the render settings (Fig.17).
I prefer an AO with height definition of the shape of the character. Therefore I usually lower the blur radius to 1. Gamma is nice to lower as well to get a broader AO. Think of it as longer raytrace in mental ray. In ZBrush AO is created with an array of depth maps though (Fig.18).
For this comp I set the AO with Multiply as the blending mode. I usually put AO further down in the layer stack and only let it affect the environment light. Letting the AO multiply the rim and key lights can make the model look dirty. In this case I just thought it looked better to put it on top of the key and rim lights (Fig.19).
The current pass can be seen in Fig.20 and the combined passed in Fig.21.
Instead of baking the cavity map into your poly painting you can create a material for this to get greater freedom later on in the comp. Create a white texture and put it in the texture slot. Copy my values and play with them. Make sure you are using a MatCap type material (Fig.22).
Turn off shadows and AOcclusion in the render settings (Fig.23).
The cavity pass was set to with the blending mode Overlay, which means that black multiplies, white adds and 50% gray does not affect anything at all. The color balance adds a red tint to the darker areas to keep a better SSS-look on the skin. Keep the opacity low as this effect can be quite drastic. This effect has a tendency to make the sculpt look sharper and brings out the details (Fig.24).
The current pass can be seen in Fig.25 and the combined passed in Fig.26.