You can get a product called Rapid Remover from most sign shops.. Although I've never used WD40 it makes since that it would work. But Rapid Remover is made to remove vinyl stickers.
WD-40 works well, but you can also try peanut butter. Yes, I know it sounds weird, but the oil in the peanut butter actually dissolves the glue and makes removal easy. It's more eco-friendly, but you have to let it rest for a while tough.