I use acetone in a needle-less syringe. You can get them at any dentist for free (just ask for one the next time you're in for a cleaning; tell 'em you want to wash out the back corners of your mouth). It allows you to control where the acetone goes with pretty good precision, and helps cut down on the risk of destroying your foams.
if you do use the Acetone (Nail Polish Remover) try not to leave it on the wheel for too long. I've found it tende to dry the rubber making it dry and crack. Not immeditaly, but if you leave it on too long, you'll see the tires dont last as long as if you never tried to remove them.