I've seen several posts asking about how to get "that dirty look" and all sorts of very good suggestions were always given, but as a result of a notice I received yesterday regarding a Hollywood costume house sale, I discovered this product called, "Schmere."
This "Schmere" says it can create anything from dirt to grass stains to sweat stains. It comes in schmere sticks where you wipe/schmere on streaks of "dirt" and something called, "dirtbags," which you pounce/pound dry powder on.
It is washable, too, just in case you made a "mistake" in how you "dirtied up" your outfit or if you only wanted your outfit to be dirty for a certain event and not permanently dirty.
What Hollywood won't think of!
http://www.lawardrobesupplies.com/Schmere-s/122.htm