The key to ammunition performance lies in storage. If you store it in a cool, dry place, in G.I. ammo cans the stuff is good for decades. All my ammo goes into the steel G.I. ammo cans that are both airtight and watertight. The cans protect the ammunition itself and allow for easy and convenient storage.
As far as use goes, arrange your cache so you use the oldest ammo first or "first in, first out." Make sure you replace the ammo that you use with fresh in a like caliber.
Another thing adding longevity to ammunition life is waterproofing. Since adopted by NATO and Combloc military ammo, waterproofing (at least in military calibers) has become common. As the ammunition is loaded, a waterproofing mixture is applied to seal the gap between the primer pocket and primer and the case neck and the bullet. Once the waterproofing mixture cures, the round of ammunition is about as impervious to the environment as you can get because both primer and powder are encapsulated in a metal "skin." How do you know your ammo is waterproof? Look at the primer and at the point where the bullet joins the case. If you see a colored ring or band at the joint, that's waterproofing. (Example: Wolf 5.45x39 ammunition has a red band where the bullet joins the case mouth and red surrounding the primer that's installed in the base of the case.) Colors of waterproofing material vary; typical colors are red, green, and black.
That's all there is to it.