There are a myriad of grunge texture vector files out there that you can use to reverse out of your artwork. Joe Diaz had a video tutorial of it showing how he aged a file to look branded but I can't find the video anymore. Do a google search for "Free Vector Grunge Textures" and you should come up with plenty of the vector files.
If you want the easy, old school method print your artwork in black and white, run it through the photocopier to copy it onto vellum. Wad up the vellum which wrinkles the paper and pops off the toner in the wrinkles. Add some scratches and spots by scraping the toner off with an x-acto knife. Scan the artwork, vectorize (autotrace) and cut. You'd be surprised how quick and realistic you can get good results.