My Aluminum 15" PowerBook G4 only gets around two hours of battery life these days, and I have to use the laptop during college lectures. I discovered a hack that can help prolong the battery life. You can hit Control-Option-Command-8 to inverse the color of your screen -- this is an accessibility option for the disabled, available through the Universal Access System Preferences panel. White backgrounds becomes black and vice versa.
I managed to get extra 30 to 40 minutes from my current battery, extending the life to 2:40. Your mileage may vary. Moreover, documents are easier on the eye when colors are inverted, and it improves the readibility under direct sunlight, such as when working outdoors.
To turn the display back, just hit the shortcut key again.
[robg adds: I can't confirm this one, other than to note that my PowerBook's estimated battery life is higher when set in inverted mode. But I didn't run the battery down for any substantial length of time to confirm an improvement.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051202125641391