Rechargeable batteries are a mystery, losing their maximum capacity quickly and without any clear cause. It doesn't help that different devices use different types of batteries, each with their own quirks for proper charging. BatteryCare (free) helps you optimize the lithium batteries which almost all laptop PCs use.
The most important thing to know about laptop battery maintenance is that partial discharges and charges are much better for the battery's life than complete drains. The problem with charging so frequently is that it throws off the calibration for Windows's battery level reading. Then your battery may read empty when power still remains, or force your PC to shut down without warning even when Windows reports power is left. BatteryCare's main purpose is to alert you when a calibration is needed.
By default, BatteryCare recommends a calibration after the battery's capacity is used 30 times. Unfortunately, the program can't perform the calibration itself. Some laptops have BIOS programs for the task, but otherwise you must do it manually, as outlined in the supplemental guide. It is a long process (around 15 hours, although you may be able to cut down the 7 hours of recommended idle time) which involves completely draining and charging the battery.
In addition to the calibration notification feature, BatteryCare provides detailed statistics on your battery, including total capacity, current capacity, wear, and CPU temperature (excessive heat can be damaging). For Vista, there are also settings to reduce power usage by disabling Aero and unnecessary background services when running from the battery.
For those who don't know about proper care for a lithium battery, the written guide may be more useful than the program itself. That said, although no individual feature is particularly groundbreaking, the combination makes BatteryCare a convenient program which will help you get the most out of your laptop battery.