Similar to my previous post on creating a Ambient Occlusion renders from Maya 2012, this technique deals with creating a plain and simple ambient occlusion render in Maya 2012 (although the same technique works with Maya 2011 and Maya 2009). To render a Wireframe image to overlay on top of your Ambient Occlusion image, follow these steps.
Or check out this updated post for adding Ambient Occlusion in Maya 2018 with Arnold.
Switch back to the masterLayer, or create a new layer that includes all of your objects.
Apply a simple lambert shader to all of your objects.
With one of your objects selected, go to the Attribute Editor and click on the initialShadingGroup tab.
Expand the Contours section and Enable Contour Rendering. Slide the Color slider down to 0 to make the color black. Change the Width to a smaller value such as 0.5.
Go to the Render Settings and under the Features tab, expand the Contours section. Enable Contour Rendering, Hide the Source, change the Flood Color to white, change the Over-Sample to 3 and choose Gaussian Filter. Expand the Draw By Property Difference and check the shown boxes. Render the image.
Now you can merge the Wireframe image with the Ambient Occlusion image in Photoshop. Put the Wireframe image on a new layer on top of the Ambient Occlusion image and change the Wireframe image's blend mode to Multiply.