I would say keep them somewhere out of sun light and as flat as possible. If you store them scrunched up for a couple of years, they'll tend to adopt that shape to some extent and be harder fit.
Apart from that, they should keep pretty well. After all, alot of them last 20 years or so on a car.
Ain't nothin' like the wind in your hair.
i helped clean out my boss`s dads place after he fell of the purch and he had some rubber bits in there for 20+ years stored in white powder called "english whiting", they looked and felt new. not sure if you can still buy it but it sure worked.
Talc? To keep the moisture at bay?
So, flat, in the dark, not bent, with talc.