Rubber-bands are simple, yet very useful. Old rubber bands(5 years?) get brittle? Why is that?
If a rubber band is usable for 5 years, it's a very good one.
There is not much You can do, just store them in a dark and cool place. Light plus oxygen is the most dangerous enemy of rubber. Nothing can be done in use.
The biggest influence has the producer, because he can choose a persistent (expensive) kind of rubber (eg Kalrez®, ask for prizes!) Of course there are less extreme rubbers in prize. Another way is the color, adding carbon black prolonges life of rubber substantially. (Protection from light) and to add some antioxidants.
The background of embrittlement of polybutadiene (polyisoprene) based rubbers is crosslinking by radical chain reactions with oxygen. Cheap urethane rubbers die from similar reactions, but those usually become sticky, even liquid by this processes.