Warm temps and high humidity. You could build yourself a humidity chamber out of a plastic storage container, some mulch and few other household items. There is an excellent response in a thread by "Citizen Insane" (George) which will give you very good information. Link is below.

http://forums.gardenweb.com/forums/load/plumeria/msg0823592832171.html

Here is a link that might be useful: Thread