You can take pictures of cracks (just with your digital camera - turn the flash OFF for this) and overlay them onto your image. Works great.

Find a crack you like that is on an even colored surface (like solid color of paint, etc - doesn't matter what color as long as you can see the crack clearly), or if it is reflective (say, a mirror, or glass, shiny paint, etc.) then get a cheap white posterboard and hold it up so the entire reflection is now a dull gray (don't worry - no matter how bright the white board is, the reflection will be dulled in the end and you can work with it).

Get that image into PS, desaturate it (Image>Adjustments>Desaturate), and change the Layer Mode to Multiply. Bring up your Levels (Image>Adjustments>Levels) - since the layer is in Multiply Mode, anything BLACK will be seen, and anything WHITE will dissappear. Adjust your levels to get the desired effect of the crack in the image you're looking for, and then either Mask everything else you don't want, or paint with 100% WHITE whatever you want to hide from view.

This is also how you can then create your own custom brushes from ANY texture/object you find and want to use later.

Hope that makes sense - good luck!
-Lew ;-)