Check out this old thread for tips and tricks, btw if you do a search you'll find several other "discussions" on the subject as well.
http://rubberstampchat.net/showthrea...=Glycerin+bath
The bath they mention is:
2 tsp glycerin to 1/2 cup of distilled water. Mix together and then pour into a dish and then leave rubberstamps to soak overnight. Make sure the mixture is shallow enough to cover the rubber but not the wood mounts and/or cushioning. Give them a cleaning in the morning, if still not soft enough you may want to give them a longer soak.