Tires
The two biggest enemies of tires are UV and ozone. To combat these they add carbon black and wax to the rubber. The carbon black changes the UV into heat and dissapates it into the tire but once the carbon black is gone, it is gone. This is what causes a tire to turn grey, as the carbon black is used up.
As a tire rolls up the road it causes fresh wax to come to the surface. This wax is the sacrificial layer that the ozone attacks. If the ozone gets through the wax it heads for the rubber next. Thats why (crazy as it sounds) it actually does a tire SOME good to use it. If the tire is stood then there is no fresh wax coming to the surface so the ozone will get through it eventually unless it is covered.
If you are storing them then keep them out of the sun light as has been previously mentioned but tread careful of the silicone based sprays that have been mentioned. My understanding is these silicone based products can actually remove the wax from the surface of the tire and leave it vunerable to ozone attack.
There are a number of non silicone sprays out there that claim to be the best and I am sure others will have their favourites. Just telling what I have been told over the years.
I found this on Goodyear's site about protecting aircraft tires from chemicals and exposure, it should be relavent:
PROTECTING TIRES FROM CHEMICALS AND EXPOSURE
Tires should be kept clean and free of contaminants such as oil, hydraulic fluids, grease, tar, and degreasing agents which have a deteriorating effect on rubber. Contaminants should be wiped off with denatured alcohol, then the tire should be washed immediately with soap and water. When aircraft are serviced, tires should be covered with a waterproof barrier.
Tire coatings or dressings: Goodyear adds antioxidants and antiozonants to the sidewall and tread to help prevent premature aging from ozone and weather exposure. There are many products on the market that are advertised to clean tires and to improve appearance and shine. Since many of these may remove the antioxidants and antiozonants, we do not endorse any of them unless the tires are to be used for display
purposes only.
Aircraft tires, like other rubber products, are affected to some degree by sunlight and extremes of weather. While weather-checking does not impair performance, it can be reduced by protective covers. These covers (ideally with light color or aluminized surface to reflect sunlight) should be placed over tires when an aircraft is tied down outside.
Store tires away from fluorescent lights, electric motors, battery chargers, electric welding equipment and electric generators, since they create ozone which has a deteriorating effect on rubber.
Cheers
Poccur