Resin releases isocyanates into the rubber, causing it to break down faster. It's generally a good idea to demold as soon as possible for maximum mold life.
BUT, you're onto a good idea---- plaster can be used to help your molds keep their dimensional stability. Then, wrap them in an airtight plastic bag (could be a garbage bag, but I use ziplocs.)
Also, keep in mind that silicones have a library life (meaning how long you can reasonably store them without them being ruined.):
Ooomo - 1year
Moldmax - 5-7 years
smooth-sil - 30+ yearswww.smooth-on.com
has some great FAQs worth reading.
Here's the one on mold storage:http://www.smooth-on.com/faq_display.ph ... d=37&cID=4