Are we talking about decals, or stickers? Technically, decals are the screen-printed ones that slide off the backing paper with either water or laquer -- like the ones that come with plastic model kits. The other sort are really stickers, or labels, or appliques -- depending on how snooty you feel. These are thin sheets of plastic -- usually mylar, and have a pressure sensitive adhesive on the reverse side.

Stickers aren't too hard to get off; all you have to do is dissolve the adhesive under the sticker. As was mentioned, Goo-Gone is good. 'Goof-Off' is better. It's a mixture of xylene and toluene, and can dissolve most petrochemical products. Be careful though -- your paint is also a petrochemical product; don't leave it on too long. Fairly carcinogenic as well, so resist the temptation to bathe in it.