Take a hairdryer to it. Set it on low/medium and hold it pretty close to the rim. If it's a tubular, be conscious of the glue and keep the dryer away from it if you can. This will ruin the decals, though - they will be stretched and ripped.

After it comes off, there may be sticky residue on the rim. There are tons of goo removers - a brand called Goo-B-Gone is pretty good. Make sure to wipe it down REALLY well afterwards.