IndyGear has tons and tons of info about distressing leather, mainly jackets:http://www.indygear.com/cow/
As far as denim goes, you can use sandpaper to accelerate wear along seams and around the various things in the pockets, but beyond that I don't know of any personal way to distress denim jackets other than wearing them.
That said, some jeans are hand-distressed, are they not? And if so, I guess that process could be replicated, but it's probably very labor-intensive. You might want to post this in Streetwear.