Dip the sticker into luke warm water for a couple of seconds and it should slide off, don't let it float off the backing paper.

That's what my mams husband (Mike) said and he's been doing this since he was a kid, all I did was cut around it with the craft knife (leave plenty of space so you don't cut the actual sticker) and do as above with warm water.

Tweezers are useful to hold it with while dipping.