At Apple Notebook Batteries, Apple tells you that you should discharge your iBook or Powerbook battery once a month. You may have thought that this is because batteries have a "memory", but that isn't really the reason. Actually, your lithium battery likes a partial discharge and recharge much more than a full - so (as Apple suggests) the ideal situation would be someone who uses there laptop unplugged on a train on the way to and from work, and then is plugged back in the rest of the day.
The reason for the once a month discharge is to synchronize the "fuel gauge". If you only have small discharges, your battery indicator will not be accurate; doing a deep discharge monthly re-synchronizes it with the battery, allowing it to give you accurate information.
I don't always take my laptop with me when I go out. To provide that partial discharge daily, I unplug my laptop when I put it to sleep for the night. In the morning, it will have discharged a bit (not much, about 10%), and I leave it unplugged a bit longer while I check mail and so on. Then I plug it back in, and let it do its small recharge.. Around the first of the month or so, I let it "run dry" as recommended above.
By the way, your battery is only good for a few years at best. If it's over two years old, you may be living on borrowed time, especially if you leave your laptop on all the time or carry it in a hot trunk: laptop batteries will last longer if they are cool.
If you are getting close to the danger point, consider buying that battery now, before you actually need it.
Visit Battery University - Portable Computing for more information on your laptop's battery.
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Inexpensive and informative Apple related e-books:
Take Control of Your Digital Photos on a Mac
Take Control of Apple Mail, Second Edition
Take Control of Your Paperless Office
Take Control of Apple Mail
Take Control of Podcasting on the Mac, Third Edition