Layers, that's what I mainly use for the effect. For the most part each image map has four to five dedicated weathering (grunge, dirt, etc.) layers, of really the same opacity, but different patterns, so I can get the effect. Some are shared, some are whatever. A "Fade" layer is merely a color based on the bitmap. I.e. I take the eyedropper tool, take a sample, then usually lighten the color a bit. Then I make the "fade" layer, then I use a 1 pixel brush and scribble the lines all over the texture, sometimes particular areas have a blob area, then I take a smudge tool and move it around, which does as the layer does, fades the texture a bit due to sun.
I got a LOMAC tutorial which basically could be applied (same image format) on my site:
http://562.50megs.com/LOMAC/Tutorials/LST.zip
Plus some help came from the templates that USAFMTL hooked me up with as well ;)