Try this: a hairdryer or fanheater. Play it on the decal, close
enough to warm it a bit. It'll soften the adhesive, allowing you to
peel the decal off SLOWLY and often in bits & pieces. you might get a
wee bit sore under the nails, but it works and it doesn't cost
anything or damage the paint (unless you've had a bad repaint & it
pulls the paint off BTDT).

I've done this on a few vehicles, including taxi decals.