There are a few main environmental contributors to cracking tires- exposure to sunlight, exposure to pollution, moisture levels and temperature. While it’s harder to avoid the exposure to pollution- especially if you live in an area that is densely populated and therefore has a higher rate of pollution, you can do you best to minimize the risks given here.
Firstly, you should try to store your tires in a cool area, as sunlight can wreak havoc with the rubber used in tires, drying out the surface and making cracks more likely. It will also sun-bleach your tires and take out the flexibility that they require. This should also satisfy the temperature aspect, since hot environments will have the same effect. Moisture levels should be kept dry but not uncomfortably so, since both high and low temperatures increase the risk of cracking.
You should also make sure that your tires are always kept above ground and with a good amount of air in the tires. If they remain on the ground for an extended length of time, it’s likely that your tire will create a flat spot in the area they are resting on. Similarly, keeping your tires at a suitable PSI will ensure there are less deformities present when you refit your MTB tire.