Sometimes leaving a hot damp cloth on the decal will loosen it up. It
might take awhile or even a few times of getting the cloth wet under
the hot water.
If that doesn't work, you can try a product called GOOF-OFF. You can
find it at Wal*Mart, in there hardware/paint section and hardware
stores. Use it cautiously. It is meant for taking marks, glue from
tape, and other junk off of painted walls and such, but if you rub too
hard and long it will start to work through the paint (water based
latex). Enamel paint seems to hold up better than water based paints.
But it can dull the area that your rubbing if your not careful. But
just buff it back out and wax.