I wrapped my receiver up in a party balloon and taped it and the Electronic Speed Controller up for use in the rain on Sunday. This worked fine but unfortunately did not stop me burning out my motor. This the first time I had a motor smoke and burn out on me and it was not that old.
Waterproofing the receiver is as simple as stretching a party balloon over it and tightening the end where the wires come out. I had already replaced my stock speed controller with one that happened to be water resistant so no action was needed there.