Here's a little cut & paste from a Tirerack article:
Since heat and exposure to the elements are the primary factors that influence a tire's aging process, drivers can prolong their tire's life by minimizing their impact. Here are some tips for storing tires that will not be used continuously.
• Don't store a vehicle with weight on its tires for extended periods of time. Long-term inactivity is more harmful to tires than short weekly drives that flex the tires and help maintain oil dispersion within the rubber compounds.
• DO NOT APPLY ANY TIRE DRESSINGS. Tire compounds are formulated to resist ozone cracking or weather checking.
• Keep the tires out of direct sunlight. The sun’s ultraviolet rays and radiant heat are detrimental to rubber. We have used a pyrometer to measure tires that were simply sitting in direct sunlight on a parked vehicle. Surprisingly those tires’ temperatures were 135° Fahrenheit on their surface.
• Place each tire in its own large, opaque, airtight plastic bag (such as lawn and garden bags) for storing. Avoid allowing any moisture and remove as much air as practical (some drivers even use a vacuum cleaner to draw out as much as possible). Close the bag tightly and tape it shut. This places the tire in its own personal mini-atmosphere to help reduce oil evaporation.
• Place the tires in a cool, dry location. It is better to place tires in a dry basement than outdoors or in a hot garage or attic. The basement temperatures will tend to remain cooler and more stable, while outdoor, garage and attic temperatures will often become hot and face daily fluctuations in temperature.
• Keep the tires away from sources of ozone. Electric motors that use contact brushes generate ozone. Keep your tires away from the furnace, sump pump, etc.
While tires will age somewhat regardless of what precautions are taken, these procedures will help slow the process compared to not doing anything.
