Do you mean extend your battery life as in the runtime of your computer on your battery, or to extend the working lifetime of the battery itself?
For the former:
1. Do as buwish says
2. Lower your LCD screen brightness to it's lowest (or lowest comfortable level)
3. Do not do intensive tasks on battery
4. Do not cover up your heat vents to prevent your fan to come on when uneeded.
For the later:
1. Completely discharge your battery at least once, if not twice, a month.
2. If storing your battery away from the computer for some time, leave it at around 40% charge - never fully charged.
3. If you're using your notebook on AC power for an extended period of time (days), then remove your battery.
4. It is better for your battery to charge it when your computer is off/sleeping, to reduce heat.