Photosourced overlays for a bit of friendly noise.
See this thread on how to make a tiling texture from photographs:
http://www.game-artist.net/forums/sh...xture+tutorial
Experiment with opacity, brightness, contrast and saturation.
You can blend a mixture together, for example you might want a different effect on the bottom section, filters can be quite cool for metallic surfaces.
Use layer-masks to vary the opacity of the texture, same rule applies in any alpha map - light emphasizes (white for solid solid in this case) and dark dulls (transparent).