1) Some people leave them empty.
2) Some people leave them with 1 second of gas fill.
Make sure you remove bb's, as state above, tension on the spring.
I personally use propane, after a game day end, I empty the propane slowly, and put in some green gas when I'm at home (After I clean them, etc.)
Then I store them in a ziplock bag, and then store it inside a container, inside a larger container.