Keep your tires away from direct sunlight and locations with high temperature, high moisture, heavy electrical machinery, welders etc. Tires should be preferably stored in a cool, dry, and dark room with controlled environment.
If possible, store tires vertically rather than stacking horizontally to reduce stress and tire distortion. If tires must be stacked horizontally, stack them symmetrically and not so high as to cause severe distortion of the tires at the bottom.
Tires are not meant to be stored for extensive periods of time. As a general rule, storing a tire for several months in far from perfect conditions will inflict damage on the tire.