Lithium Ion notebook batteries wear down because of two factors: 1) active usage in your notebook battery and 2) natural aging of the notebook battery. Both will wear down your notebook battery over time; the trick is to minimize their impact while still getting the performance out of your laptop battery that you need.
The most important thing to understand about laptop batteries is that they are always losing a small bit of their charge. The hotter the temperature, the faster notebook batteries loose their charge. So rule number one is: keep your notebook battery cool. Notebook battery manufacturers store their products at around 60 Degree F. (It doesn't help to put them in the refrigerator, and you can damage a battery by freezing it.)
The second most important thing to understand about notebook batteries is that their capacity decreases with each cycle of charging and discharging (or usage). By itself, this is not surprising - but when combined with the previous point, it leads to a surprising conclusion.
When laptop users leave their laptop battery inside the machine but leave the computer plugged into the wall, the laptop battery is going through a constant charge-discharge cycle. The notebook battery is sitting unused inside the notebook, discharging a little faster than normal because of the notebook's heat. Once its charge level drops to a predetermined level (which is different for each manufacturer), the AC adapter provides extra juice to "top off" the notebook battery. As the laptop battery gets older, it tends to self-discharge a little faster, which accelerates the process even further.
Lithium ion notebook batteries normally offer 600 to 800 charge/discharge cycles over 1 to 3 years of useful life. When you use your notebook battery as described above, you are needlessly using your supply of recharges.
One additional note: many people recall that older notebook batteries on early computers worked best when they were fully discharged before being recharged. While that remains true for Nickel Cadmium technology, today's Lithium Ion notebook batteries work best when they are recharged when they still have 10 to 20% capacity remaining. So if you are using your notebook battery on a long flight, try not to get in the habit of using the notebook battery until it has almost no charge. Your laptop battery literally won't be the same when you re-charge it the next time.
Some Guidelines for Notebook Batteries:
Remember that new laptop batteries generally arrive in a discharged state; you must charge them up for at least four hours before their first use.
Keep your laptop battery cool, but do not put it in the refrigerator or freezer
Don't use the notebook battery if you don't have to! Many people keep their laptop plugged in most of the time. They can preserve their notebook battery life by storing the battery outside the laptop in a cool place.
Try to avoid running your laptop battery all the way to zero
Don't charge your notebook battery before long periods of inactivity
Much as we'd like to sell you more laptop batteries, don't buy spare notebook batteries for later use. The longer you hold them, the less useful life they provide.