Cool dry dark storage, away from sources of ozone... Depending on where you store your bike a saddle bag might qualify. I have 6 year old butyl tubes that are perfectly fine. I have no recent similar experience with latex tubes, though some say they are more subject to deterioration over time and I have seen latex surgical tubing and similar deteriorate. I keep the spare tubes on my bikes tightly wrapped in plastic wrap to keep the compressed to a compact size and protect them from abrasion. It may contribute to preserving them to some small extent.