Another method you can use to eek out a bit more life of a battery (if it's of Li-on composition) is by resetting the recharge memory. I learned recently about Lithium Ion batteries having a sort of 'recharge memory', and it's specifically because of how the chemicals react.
If, on a full charge, your battery lasts an hour, and you plug it in and let it charge after ten minutes of use (using up 1/6th of a charge), then you'll start a process of changing the retain capacity of the battery. If you continue to only let the battery use up 1/6th of a charge before plugging it in, then the Li-on battery will start to 'think' it's charging fully, when it's only charging that 1/6th. After about 15-20 charge cycles like this, letting it die completely and plugging it in will cause the battery to charge up to the 1/6th point and then 'think' it's full. It'll stop charging and you'll only have ten minutes before the thing is dead. Most people will then tell you that the battery is shot and you need to buy a new one.
However, there is a way to reset the Li-on's recharge capacity, to force it to 'rethink' how full it is. Charge it up as much as possible, then unplug it and let it die, use up the entire battery. Charge it fully, and immediately unplug it and let it die. If you keep doing this whenever you have the time to over the next few days, you'll notice that the battery will last longer each time you unplug it. If you keep the habit of only plugging it in when it's completely dead, then eventually you may end up with a battery that lasts LONGER than it did initially!
I have a battery meter gadget for Win7. It logs what your initial battery capacity was in Watt Hours (Wh), your current retain capacity, and the difference in percentage. On my laptop, right now, it's telling me that my initial retain capacity was 360 MAh (or 4 Wh), and my current capacity is 400 MAh (4.3 Wh). My battery currently can charge up to 111% of it's original capacity!