Re: old powders
All very good answers here, and moisture really isn't the bugaboo that you may think it is. Powders hygroscopic, and tends to absorb and release moisture pretty readily. As odd as it sounds, much of the manufacturing process actually takes place in water, and some long term storage of archive samples are kept under water. read something a while back about such a sample of the original run of Bullseye from over 100 years ago that had been stored thuis way, and when it was dried and tested, still met the specs. Not recommending storing it in damp environment if you can help it, but it's actually heat and temperature extremes that are the things to watch for. Airtight packaging (like the ammo cans already mentioned) are good, but they'll still need to be kept somewhere that doesn't experience wide temperature variations. Had a friend who used to keep his powder in an old refrigerator (unplugged, not running), simply because the temps remained so stable due to the heavy insulation. Since he'd probably forgotten more about this stuff than I'll ever know, I listened, and now keep mine in a similarly stable environment.
Kept this way, a powder's shelf life is almost indefinate.
Hope that helps