Critter made an awesome thread on something similar. These are ways to conserve your battery while you're actually using it.

Lower the backlighting on your screen. Use a brightness that doesn't strain your eyes, but also one that you don't need shades to view either.

Turn off the wireless signal...when you're not using it that is. If your laptop's wireless is on it will no doubt keep looking for that signal....this can use up to 20 minutes of your battery life on its own!

Turn off that device! As long as you're not using them, and while you're disabling things anyway....you can also disable these: ethernet adapter, infrared transceiver, and your bluetooth radio.

Disable startup items. If you have several things that are set to start up at launch, disable them for optimum battery usage. Not only does this use battery power, but it makes your computer run slower too.

Condition your battery. Yes, like your hair, your battery needs some deep conditioner. You have to sort of "train" your battery to run how you would like it to. This is a bit more complex, but let me break it down for you. Say you run your laptop on the battery alone for one hour, and then you plug it in to recharge. You just conditioned your battery for running for less time and needing more charging, especially if you do this often.

Teach your laptop a new trick. Try letting it drain completely without the assistance of the charger (unplugged). Then charge it all the way back up again. The more often you do this the better battery memory your laptop will have. Ta-da new trick learned, get yourself a treat.

Defrag. Making more room on your hard drive lets your computer work at a quicker rate. The less your computer has to do, the more battery power you save. Learn more about defragging here.

External devices too. You'll have to stop plugging everything into your laptop. Use the mousepad your laptop came with...jeez. You can save so much power by ditching some of those devices you plug into to your laptop.

Multitasking is for losers. Try to do one thing at a time when you're just on battery power. Too many things running at one time pull the battery's power.