There are a couple of tricks. One is to use a warm moist cloth, allow it to sit on the decal for a while then try to lift it - use a bit of low-tack tape if required.
Raised by wolves, tamed by nuns, padded for your protection.
Terry Miesle
Never trust anyone who says they don't have a hobby.
Quando Omni Flunkus Moratati
Depends on if you sealed with clear/dull coat or not. If you have not seal it with a coat, simply take a masking tape and tape it on the top of the decal and slowly life the masking tape. I tried many times and this works fine with out lifting the paint.
if you seal it with clear coat, best thing to do is the sand that part off and repaint the area.