Below is a render without ambient occlusion.
And here is one with ambient occlusion. The image looks less flat now and shows more depth (in particular, the white wall near the hedges).
To make a render layer for ambient occlusion, first click the below buttons on the channel box to get to the layers window.
Click on the Render radio button to view render layers. Select geometry in the scene that you want to apply ambient occlusion to and press the button highlighted below to create a new render layer with selected objects in it.
Now in the Hypershade, middle mouse drag the Surface Shader into the Workspace.
Now left click on the Create Maya Nodes tab and switch to Create mental ray Nodes.
Expand the Textures tab on the left hand list and middle mouse drag the mib_amb_occlusion node onto the workspace.
Middle mouse drag the mib_amb_occlusion node onto the Surface Shader node and select default. You’ll see an arrow inbetween the two nodes to show that they are now connected.
Now in Render Settings, set the renderer to be mental ray. Go to the Features tab and tick Ambient Occlusion.
When you render the render layer, you get a black/white image that you can superimpose on the original image using the Multiply mode. If the render doesn’t look quite right, try changing the max_distance parameter of the mib_amb_occlusion node or increase the number of samples to make it more smooth (both in the attributes editor).
Ticking the Final Gather checkbox in the Features tab of mental ray preferences also greatly improve renders.
Here is a render without final gather checked.
And here is one with it turned on. Notice the dark black areas in the shadows earlier is lit due to simulated bouncing of light.
The render is a bit too bright now that light bounces around, so override light intensity for the render layer.