sand probably wont work effectively, i was thinking for my trails maybe a waterproof cover to put on them... or even just some big boards that could be lifted off before riding.
carpets! just stick carpets on ur jumps and u can ride when its damp and stuff.
i live in singapore and it rains pretty much everyday. but i still get to ride all the time because of the carpets. its just the run in thats a pain in the ass to ride when slippery
at a track near us all the berms got filled with water, what soem guys did was shove a pipe through he bottom of the berm directing the water behind the berm, and filled up the hole with stones so it was still rideable. i guess you could kinda do the same thing