Several basic tricks....by the way, why don't we take this to the Texture Forum? It's been lonely over there! Anyhoo...Layers. Work in layers, lots of layers. If you can, use more than one tile, with different rates of tiling (so one repeats every 200 pixels, another repeats every 300 pixels, so the combination doesn't repeat for 600 pixels.) What I did for my last texture set is start with the largest scans I could (so there was less tiling), then I created multiple adjustment layers with hand-painted changes in saturation and hue and lightness, as well as hand-painted details like seams and rust spots and loose threads et al. Many of these layers were just cloud filters set to introduce subtle splotches in the base texture; that alone will break up a large expanse of tile very nicely. The other thing is to have a real sense for how things wear in the real world. Do a lot of looking and take a lot of pictures. If you are adding some mud to a riding costume you don't add random spatters over the whole thing; you think about how the front hooves throw dirt back against the front of the lower legs with some force, and spattering with less velocity upwards along the front of the rider, until there's only a spot or two on the hat. Etc., etc.!