Exercise Your Notebook's Battery
Early portable computers used NiCad batteries, which were subject to the dreaded "memory" effect. To get maximum run time from them, you had to discharge them completely before recharging them; if you failed to do this, even a fully charged battery might suddenly run out of power after a short time.
The lithium-ion batteries used in modern Mac notebooks (and iPods) don't suffer from memory, but according to Apple they still need to be "exercised" (that is, discharged and recharged periodically) for maximum performance. If you leave your notebook plugged in all the time, the battery never discharges beyond a few percentage points, so it gets insufficient exercise. For such users, Apple recommends that once a month you unplug your computer, run it on the battery until it discharges completely, and then recharge it.
For more information about Apple's notebook batteries, check out www.apple.com/batteries/.