Tire manufacturers (I've dealt with most of the major brands over the course of my career) recommend storing them flat, not upright. If you have it, put cardboard or plywood under the bottom tire and between all the others. Store where the temperature is as constant as possible, low humidity is good, and NOT near electrical motors - they emit ozone while running and ozone is very bad for tires! Do not put silicone based protector products on them (ArmorAll, etc) as silicone is also bad for tires, use water based products (not sure of any specific brands other than from Griots Garage). If mounted keep them inflated to about 2/3 to 3/4 of their normal operating pressure and rotate the stack (move bottom to top, etc) to prevent any problems. Last, but not least - make sure animals can't get to them! I had a set of expensive tires chewed up during storage to the point I had to get rid of them (over $1,000 down the drain)! Hope this helps.