Caulk the base of the nozzle to create a perfect seal
(Do this before using it so you do not expose the unused silicone.)
I've had this stuff store for more than 2 years using this technique.
I tend to do this before puttingloading the nozzletube into the gun but you can do it after just the same. You don't need a lot and you can push out a little caulk with pressure from your fingers after cutting open the tube. You apply a slight amount all around the base of the nozzle and the twisting motion will spread it evenly to seal it.
If stored for less than 6 months, you may be able to just grab the hardened stuff and pull it right out in one piece. I've had mixed success, but sometimes it breaks and you'll have to push it out.
After 6 months of storage, the caulk in the nozzle will harden about half way down and you'll need to take it off and push the hardened stuff out (or you cut a much larger hole in the nozzle), but otherwise the rest of the tube is brand new.
This has the major advantage of not causing caulk to cure inside the tube because there is zero exposure. When you're done, wipe off the excess and store.