Head down to your local home store and get some Xylene, it's by the paint thinners.
It's a slow evaporating solvent. Put some on a rag and get the old adhesive wet and let it work for a bit. Keep rubbing with a wet rag, eventually it will let loose.
Lacquer thinner or acetone may work but because it's fast evaporating it doesn't get time to work.
You can also get some plastic razor blades or get a plastic putty knife to help in removing it once it gets soft.
You could also get a decal removing wheel.
I own a cleaning company and have been doing this for 27 years, the Xylene is your best bet.
But, always test advise you get in an inconspicuous spot to be sure it won't damage anything.