In real life, I get around 3 hours of battery life. One of the simplest things that you can do to extend battery life is to reduce the brightness of the screen. To further extend battery life, reduce processor speed and turn on processor cycling. Also, upgrade to the latest PowerBook Internal Modem extension off of Apple Computers support site. I you were using the modem and you put the PowerBook to sleep, the modem would remain powered up and eat up some battery life.
Sleep mode Watch out for sleep mode.
I never shutdown my PowerBook. Make sure that when you close the lid on the PowerBook that the light starts flashing. I once closed the lid expecting it to go to sleep, but some program somewhere prevented it. I drove two hours or so, reached in my laptop case and just about burned my hand. The computer was left on and laptop cases are good insulators.