Your absolute best bet is if you can peel MOST of it off, then use the sticker itself to remove the rest.
Goo Gone works well, but it's a little...greasy. Honestly, if it's bad enough that the sticker method won't clean it up, then I try to find another case I can swap it out with (I keep some spares, if I ever trade in a game I swap it into one of the crappy cases first).