Here's a quote from the Wikipedia howto:
Unlike NiCad batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a longer time, they should be brought to a charge level of around 40%. Lithium-ion batteries should never be "deep-cycled" like NiCd batteries. Li-ion batteries should be kept cool. Ideally they are stored in a refrigerator. Aging will take its toll much faster at high temperatures. The high temperatures found in cars cause lithium-ion batteries to degrade rapidly. Lithium-ion batteries should never be depleted to empty (0%). According to one book, lithium ion batteries should not be frozen. Note that most lithium-ion battery electrolytes freeze at approximately −40 °C, which is much colder than the lowest temperature reached by most household freezers. Li-ion batteries should be bought only when needed, because the aging process begins as soon as the battery is manufactured. When using a notebook computer running from fixed line power over extended periods, the battery can be removed and stored in a cool place so that it is not affected by the heat produced by the computer. (However, a notebook computer's battery prevents sudden loss of the data in memory during power failures and brownouts. Reasonable alternatives are the use of an older lithium ion battery or an external uninterruptible power supply.)
Notebook computers. At this point in the 21st century, they have become an indispensable part of many people's everyday lives. From the commuting business executive and the busy college student, we all want to be mobile with our computers. The big drawback of that is the fact that there is not always a plug around when you need one. A notebook computer's battery is its life-blood. Without proper care, your notebook battery could fail much earlier than anticipated. By following a few simple steps, you can expect to get the greatest amount of usage possible out of your notebook battery before a replacement is needed.
1. Upon purchase, charge the battery to full capacity. Whether you have purchased a new computer have purchased a new battery for your laptop, charging it fully as soon as you open the box is the best recommendation. This allows the battery to reach a full capacity before you begin to use it "unplugged". Most batteries come with a partial charge, enough to get you going if you absolutely had to. Giving that battery a full charge when it is brand new will help set the charge capacity for the battery at its highest possible level.
2. Once you reach a full charge, leave the computer plugged in for at least 2 hours. This action lets the battery "rest" at a full charge for a fairly lengthy period of time. At this point, the battery is not drawing power from your wall socket to charge, and it is also not supplying any power to your system. Letting your battery rest for a time after a full charge helps to "lock-in" that full charge capacity.
3. Unplug the computer and drain the battery. It might sound silly, but now that you have gotten that battery to a full charge, you need to drain the power almost completely. Again, this will help determine the charge capacity of the battery. Leave your computer unplugged and use it until you get a warning telling you that the battery is almost completely discharge. Then save whatever you are working on and power the computer down. Plug back into the wall socket for a fresh charge. Then use the computer at your leisure.
If you follow these steps about once a month, your notebook battery should last a nice long while. Not to say that you will never have to replace it, but at least it will be a long time before you need to.
Info about our guestwriter of the day: Michael Paul is the host/producer of the TechCast Weekly Podcast, a popular weekly show dedicated to providing computer help, tips, and tutorials to help make you computing life easier and easier to understand. Visit http://www.techcastweekly.com for the latest shows and tips for the average computer user.