Yo Kass answered
Is there any chance you could simply cover it up with a more mature design?
If it's fairly small, uses light colors, or has started fading - then a cover-up would be the most convenient option.
I know you're trying to put that whole 'tattoo phase' behind you, but maybe its possible to find a design that's elegant and more 'you right now' as opposed to 18-year old you...

Your other option is laser removal.
Laser tattoo removal
A friend of mine is currently going in for regular sessions to remove a large tribal design from his arm, and it is:
- costly (here in the UK prices range from £50-£200 per session)
- painful (almost as bad as getting another tattoo done)
- time consuming (most tattoos require 8-10 sessions)
- will look pretty bad until it is removed:

Tattoo removal cream
There is a third option: Tattoo removal cream. This is meant to work by latching on the pigmentation, and then getting your body to reject it.Unfortunately, the medical repercussions of that aren't 100% certain, and some doctors have spoken up against it - but it is something you could investigate!