One thing that really helps is to think about how and where a costume gets dirty and worn. I've used the sandpaper method as well as a cheese grater (the small holes) on hems and cuffs where there is typically more wear.  I've also load up spray bottles with a concentrated RIT dye mixture and hit the appropriate areas with color. Yellow for sweat around necklines and armpits, brown for dirt at knees and elbows, black or gray for general grunge, green for grass stains.  Once when renting some solider costumes in college we couldn't use any permanent methods so I was sent out to a big pile of dirt from some excavation work to simulate dirt with . . . . dirt.