Keep the electrons flowing for long battery life

Apple has some good advice for maximizing the lifespan of lithium ion batteries:

For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her MacBook Pro on the train, then plugs it in at the office to charge. This keeps the battery juices flowing. If on the other hand, you use a desktop computer at work, and save a notebook for infrequent travel, Apple recommends charging and discharging its battery at least once per month.

Temperature is also important; batteries prefer room temperature as much as humans do. Keeping a rechargeable battery on a charger all the time heats the battery and reduces it's useful life.