How to set up an Ambient Occlusion from scratch.

First, you need Mental Ray. Open your Hypershade and have it show Mental Ray nodes. Scroll down to textures and middle mouse drag the "mib_amb_occlusion" shader to the black area on on the bottom of the Hypershade. Then go back to Maya nodes and drag out a surface shader to the black area. Connect the "mib_amb_occlusion" out color to the Surface shader's out color. Now it is just tweaking the spread and max distance. It will vary scene to scene so i'm not going through it for you.

one last tip is you want to make the camera environment white.

To do this select the camera you want to render from. Open the attribute editor and scroll down to environment. Open up the box there and slide the slider to the right.

The render setting are in the picture above.