I use this trick when I'm pouring salt baits. Before you use the salt, or before you grind it, put it in a pot and heat it up on the hotplate. This "cooking" will eliminate all of the moisture that the salt has attracted; hence, it won't wind up in your plastic.
Since I have been doing this I don't get moisture in the plastic. I heat the salt even if it dosen't feel damp.
Give it a try, I think you'll like the results.