How to extend shelf life of already opened one-component silicone sealant or glue?
Much like the bolt suggestion, I found these at a hardware store several years ago and have used them regularly.  They are actually called caulk savers.  Unfortunately, I don't recall exactly where I found them, but I have seen them around from time to time.  I did use them to save some silicone at one point, but only for a few days, maybe a week or so.  I doubt it would work any better than a bolt or screw.  I don't have any scientific basis for the next thought, but my suspicion is that once silicone contacts air, it starts a slow irreversible curing process.

One other potentially useful bit of information I stumbled upon a while back was a tip for clearing a stopped up caulk tube nozzle.  Simply take a utility knife and split the nozzle long-ways.  Clear all the dried material from the split nozzle.  Then just close it up and wrap it tightly with electrical tape.  I have done this several times and it works great.