I first start with a regular A&D shader and apply it to my scene. In the diffuse node I applied an image that I created that looks like dirt, grunge, and scuffs. You can scale the map by playing either with the scale in the map itself or by creating a UVW modifier to the scene. I prefer the UVW modifier. The reason I do this step first, is just to get the scale of the map looking right in my scene.
Then I create a mental ray material, and in the surface slot I add a mix map. In the Mix Amount slot is where I add a Ambient/Reflective Occlusion map (I like to set my Max distance to 3' usually). Now that the AO map is in the Mix Amount slot, Color #1 and Color #2 are being rendered but controlled by the AO map...so you can now put anything in either of the color slots.
Because I wanted to render a grunge/dirt pass, I copied and paste/instanced my map that I created into the Color #1 slot of my Mix map. Now you can apply the shader to your scene and render.
This shader will only render the image in the areas that are occluded or in close corner areas of geometry, and give the effect of dirt and scratches that are in these tight spots, much like real life.