This is what the iPhone was made for. INK: Tattoo Simulator will save your ass (literally) from desecration by a massive tattoo of an obscure Star Wars character, the name of the girl you just met in your freshman college biology class and want to spend the rest of your life with, a portrait of Newt Gingrich or whatever kooky longing for ink your drunk mind might come up with.
Of course, you could also use it (far more effectively) for simply repositioning a tattoo when you’re sober, but that’s hardly as interesting.