I have created scenes in Poser using graphic model cars in parking lots or on roads and freeways. They do not look very realistic because they all looked like they are too clean and shiny and were driven off a showroom floor. This also goes for aircraft,boats or anything in a outside evironment.

I was wondering if I could get some advice or someone to point to where I can find information on how to modify any of the models supplied textures to make the props look more like they would really look. 

For textures its obvious that one could import the textures in Photoshop and smudge up some layers with a dirt brush, but I am sure there must be some technique involved to get adequate results. Then there are procedural textures! --- how would you handle that??? 

Any advice and infromation would be much appreciated