I've been motor spraying them and lighting them on fire lately...that seems to soften the rubber a lot!
Generally we've been finding that:
Motor spray works best with ProLine tires
Paragon in the black can works best with Panther tires
Death Grip "foam" works best with Losi tires
But a good cleaning and hit with simple green after each run goes a long way in tire performance so...whatever works for you. For best results over the life of your tire though, keep them in a zip lock bag when you're not racing...so, between weekly club races, toss them in a bag.
TQ Racing Wire