From Apple’s Maximizing Battery Life and Lifespan page:

If you want to store your device long term, two key factors will affect the overall health of your battery: the environmental temperature and the percentage of charge on the battery when it’s powered down for storage. Therefore, we recommend the following:

• Do not fully charge or fully discharge your device’s battery — charge it to around 50%. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. Conversely, if you store it fully charged for an extended period of time, the battery may lose some capacity, leading to shorter battery life.

• Power down the device to avoid additional battery use.

• Place your device in a cool, moisture-free environment that’s less than 90° F (32° C).

• If you plan to store your device for longer than six months, charge it to 50% every six months. Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it.

This is one of those tips that applies widely, to all lithium ion devices. Best thing you can do is find an active home for your old devices. But if you can’t do that, and want to keep your battery alive, follow the advice above.

[Via Reddit]