do not use WD40. use 100% pure silicon spray. boiling them in water does soften them but you will need to try to rejuvenate the rubber compound. depending on the age and cracks, etc on the tires, the silicon may or may not work.
Spray liberally with 100% pure silicon spray (get this at your local autoshop) and them place them in a ziplock bag and let them rest to allow the silicon to (hopefully) soak in.