Heat is the easiest way to remove decals, but sometimes a hairdryer won't have quite enough oomph for the job. I use a Craftsman heat gun on the 'low' setting when replacing old registration decals or vinyl lettering.
Another option is a power washer... just be careful as any device (heat gun or power washer) used improperly can cause damage that you never intended.
Good luck!
- Kevin ><((((*>