Points about battery life from macintouch.com:
1. Exercise the battery monthly. They stagnate if not used regularly. Battery University states the exercise is needed after every 30 charges (cycles). Apple is likely trying to simplify so I go with the 30 (partial) cycles as I don't use it as frequently.
2. If not to be used in the near future, discharge it to 40 - 66% and store in refrigerator (NOT FREEZER; keep above 0°C/32°F). I keep my extra battery there and warm it then charge it for flights.
3. Never allow to fully discharge and leave at that state. This is what likely killed the camcorder battery for Herman. Canon (mine is a ZR50) states to remove the battery from the camcorder when not in use as it will discharge it even if in off position.
4. Avoid leaving it plugged into the charger for prolonged periods such as sitting at a desk for weeks. Though I believe it's the least problematic, Li-ion batteries don't like being at a fully charged state for long periods either. As Herman said, it's best to use them or they will crystallize and lose life.
It is useful for laptop battery, camcorder battery and more.