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This is another blog post for digital artists pretty much exclusively.
Recently while learning about procedural texturing I saw an interesting use of Ambient Occlusion. The point was made that objects tend to develop rust and grime in areas that are partly, or completely occluded.
Since adding rust and grime is a big part of adding realism to textures, I set out to find ways to apply this to traditional texturing.

The basic process is to first adjust the curves of your AO map so there is little grey, you don't want the rust/grime fading in and blending with the base color, you want it to appear to be on top.

Once you've got that looking nice, Use the adjusted map as a layer mask. (To paste the map directly in, you have to ALT-click on the layer mask to open it in 'document mode', then ALT-Click again to get back to your regular project.)
Here's the AO map, and the resulting texture from this process.

Clearly this is far from completed, but it's a quick and easy way to lay in some detail in the right areas. Obviously you'll need to go in and do some brush work to make it look more realistic. Adding some noise to break up the AO map (render clouds set to overlay...) is one possible solution. I think this could be very usefull if you needed to texture a lot of background props very quickly. I'd be hesitant to use this method on a main prop, unless it was just to block in the details in a first pass.