Setting up AO Materials for a Render Layer in MAYA: Mental Ray

This is all using Mental Ray's mib_amb_occlusion node.

It should also work with V-Ray's Vray Dirt node if you're using it.

First Method - Surface Shader

Simply make a surface shader (under 2D textures) and connect the occlusion node into its Out Color.

Second Method - Mib_lambert_illum

Use the mib_lambert_illum material (under mental ray textures). Set the ambience (top) and diffuse (bottom) slots to full white, and the 'ambient' slot (the middle one) to plug your mib_amb_occlusion node into.
(When baking out an AO map to multiply over your diffuse layer in photoshop, using this method seemed to work more often for me than the surface shader method. I think I gave it 128 or 64 samples for a 1K map).

Third Method - Lambert -

Use a normal lambert (the MAYA texture one) and set all of the slots to be black. Plug the mib_amb_occlusion node into the incandescence slot.

Fourth Method - Mia_material

Full White Diffuse color. Black for Reflection. In the Ambient Occlusion section in the attribute editor, Ambient Shadow color is black, Ambient Light color is white (default is the other way around if you want the material to have ambient occlusion without interfering with its diffuse and etc because the Ambient Light will wash out diffuse if it's not set to black)

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Also important that there's no lights in the render layer (hide them if there are any). Unless you are using the surface shader method because surface shaders ignore lights anyway.

Also note that if the result is going too much into middle gray (where even the lightest parts are grey) you can pump the value of the Light color in the ambient occlusion up. (or lower the occlusion distance if it's shadowing too much all over)

- when choosing the color and setting it to white, you can change the way to set the color at the bottom right. There's a dropdown from setting the color by "RGB 0 to 1.0" and change that to "HSV".

With HSV on, now you can pump the value of white past 1 and the lighter areas'll whiten right up. The last project I did had to have my ambient occlusion pumped to about 3 ish to make it white enough for my liking.

Be careful with Black going into negative values, though. It might burn the look, so it may be better to just adjust the image sequence in after affects (or photoshop for the baked out map) with a quick levels adjustment. The key in Maya is getting the right kind of shadow falloff.