Alright, most places, unless you *know* the guy who's tattooing you will make you sign a form to the effect that you're completely sober, voluntarily getting this tattoo, don't have any bloodborne illness (HIV, Hepatitis), and you won't hold them responsible for anything.
In short, I wouldn't do anything to numb the pain of a tattoo. It doesn't hurt that bad, unless you're getting it on a really sensitive area like the ankle or inner thigh.
You get used to the pain after 5 or 10 minutes. If you're doing a really big piece, chances are your tattoo artist will break up the work - most people I know don't like to spend 8 hours tattooing the same guy. There's a 2 week healing process in between sessions, and I'd definately keep the drug use to a minimum for the first couple of days after you get it.