I've literly taken an old screw driver, heated it up red hot, then poked holes in fabric to make a more uneven hole in some fabric. I've aso used a wire brush bit on a drill too, to wear fabric. Rit dyes work well too, and can usually be controlled a bit by the temperature of water you use. The cooler the water, the lighter material will pick up the color, the hotter, and the darker it will get with the same color dye.
It also depends of the fabric. Cotton will take color very well, cotton poly blends still pick up the color, but not as dark. Polyester, that won't dye using a water based dye.
Painting works well too, you can dilute fabric paint, and spray it on with an air brush, or you can use spray paint and mist the material.
What ever you try, use a piece of scrap to test before you try it on any costume.
You want to make sure the most "wear" is at the areas that would naturally wear - knees, elbows, collars, cuffs, etc. The same holds true for dirt - you wanna have more dirt where it would naturally accumulate - sweat stains under arms, around collars, etc. I've seen very good weathering techniques ruined by applications that were too uniform, too random, or too illogical (i.e. - most people won't rip their shirts in the small of the back).
