There are plenty of battery life extension tips articles if you'll search. The first result on my search was one of the more comprehensive and concise guides.
I'll summarise what seem to be the most repeated points:
- Use your resources as lightly and efficiently as possible. Everything comes back to this point.
- Turn the screen brightness down.
- Don't run resource-heavy programs, don't run virus-scans and run as few programs as you can at a time (meaning you might want to shut down some taskbar apps). If you use Windows Vista, disabling Windows 3D Aero might take a chip off your total resource consumption.
- Minimise your usage of the CD/DVD drive, Wireless card and external devices. If you have an external hard drive, copy the files you want to use onto your local hard drive first.
- More RAM and regular defragging both decrease the demand on your hard drive.
- Utilise Power Options in your Control Panel to keep system resources in check.
- Like Kara said, keep heat low so as to keep the system running at peak efficiency.
If you live in a hot climate like myself, I'll throw this in: A friend of mine had significant heat problems cooling pads weren't solving. One miniature 5" fan pointed at his laptop's exhaust vent worked wonders.