I store mine out of sunlight and away from electrical motors. Electrical motors generate ozone which apparently will deteriorate tires. It's probably also a good idea to rotate them from time to time so they're not sitting on the same contact patch.
Get two pieces of cardboard lay a tire between the two pieces of cardboard. Tape the two pieces together real tight hopefully with a gap between the tire and the floor. Stand them up in the corner and forget about them until you need them.