I had an opportunity to listen to Claus Christensen's (from Tropica) advice about this recently. He suggests an absolute minimum of water, shipping them moist, but definately not damp. He will often refrigerate plants while on collecting expeditions and reports that many of them will do fine for up to 4 weeks when stored in this manner. The idea is to slow the rate of metabolism.
One very good (IMO) internet site recently shipped several varieties of stem and rosette plants to me essentially dry, alone in air-filled ziplock type bags. Everything including Didiplis and R. wallachi arrived in perfect condition after 4 days in transit.