How should I store inner tubes to maximize their shelf life? The LBS's in my area tend to have major sales around the same time once or twice a year. During this time, I can get replacement tubes at about half their normal price. I can buy several tubes, enough to last a year or more.

What is the best way to store the tubes so they last as long as possible?
Dry and cool place is the way to go. But also play attention to other items stored near the tubes. Do not store them near paint or solvent containers. Many chemical products can damage tubes, even just the vapours. Anything like gasoline or other fuels should be avoided. They chemically degrade rubber and can easily render them useless. Gasoline vapour for example, over a year can turn a tube into a black paste stain...

Other factors that affect rubber on the long term are UV light and oxigen or ozone. Thus you should also avoid storing them exposed to direct or strong sunlight. UV light degrades rubber so it becomes dull looking and less flexible. When it happens to a tube, it does't show until you inflate them, and it looks as cracked or notoriously porous surface. 

It also helps to store them in a somewhat thick plastic bag and tuck them tight (Ziploc type would be overkill there) specially if they come packaged in plain cardboard boxes. Some high powered electric motors, generators or dynamos produce sparks in their brush and collector parts (i.e. electrical connection between moving parts). These sparks are known to produce ozone, so, if by any chance you have such equipment, do not store the tubes near it. (High speed lathe, electric grinders, etc.)

Personally I have had sucess storing used tubes for very long periods (some more than 3 years) by puting them in a thick plastic bag in my closet. I usually apply talcum between the tire and the tube, so these tubes where stored all covered with talcum. I think it helps because previous attempts, I washed the tubes before storing, and they ended stuck with each other and where damaged when I tried to separate them; they where otherwise in similar conditions.

However, in my tropical country, temperatures are almost all year between 15 and 30 ºC (60-87 ºF) so I have no clue on extreme temperature effects.