What is the proper way to store tires to prevent cracking? I have several extra tires, two on a spare wheelset, and others in a bundle.  I am not riding as much as I was a year ago, so I will not be wearing out tires as fast as I was.

What is the best way and conditions to store these tires so that the rubber doesn't just crack and split when I put them on wheels and pump them up?

Note: I don't know if it makes a difference, but I live at about 5000' of elevation in a very dry climate; forced air heating is used in my apartment about 6 months a year.

More info as an example: I had tires mounted on wheels, deflated, hanging from ceiling hooks.  I don't know how long it was that I hadn't used them but it was probably 12+ months.  When I inflated them a week ago to the recommended pressure the sidewalls showed cracking parallel to the rim on the whole, or nearly the whole circumference of the tire.
A lot of this is guesswork on my part. 

Entire bike: I would deflate the tires, keep them on rims if possible, and hang the bike up. Putting weight on the tires when they're deflated is probably bad for the tires -- and for the rims. 

Rims with tires: I would follow zigdon's answer and hang up the rims and deflated tires. 

Bare tires without rims: I'd store the tires on their sides, not stacked too high. Perhaps putting them in something like a pizza box, or between pieces of cardboard, would help avoid the tires crushing each other. 

In general: I would also flex the tires every so often, to stop them from getting brittle. Also, ozone is bad for tires, so I'd not store them near anything that generates ozone (furnaces or electric motors), and I'd store the tires away from extreme heat or cold.