I have the same problem with reef crystals, even tho it has the rubber seal on the 5 gallon bucket and I whack it with a hammer to close it. I live in So Cal, dry as a bone, and I store the bucket in a different room from my tank. I decided to just by the 50 gallon bags and use them up before they get "caked" instead of the 160 gallon bucket. More money, but then I get through the salt before it gets caked.
Do you use the salt that has hardened, by breaking it up? Or do you throw it out?