If you're running OS X 10.2.2, open system preferences, click on network. It'll bring up a prefs pane for your current location. where it says "show" there's a drop down list, select network ports. Then disable the ones you aren't currently using, like airport or ethernet. I've also made a battery only location with all network ports disabled for sitting around cafe's.

other tips: get as much RAM as you can stuff in the silly thing. Cuts down on hard disk "virtual memory" paging.

if you're running an Intel series processor, go into the BIOS and make sure CPU throttling is enabled. This will slow down your processor during non-intensive tasks. Since a 100mhz CPU is more than adequate for displaying your system clock, you'll probably never notice the performance difference.

If running Win98/ME, get "RAIN" (unnecessary on WinNT/2k/XP). RAIN will shut down your processor when it's not in use. It leaps back to life so quickly that there's no speed degredation.

On a Mac OR a PC, use Norton Speeddisk to keep your hard disk compact and tidy; again, this will cut down on hard drive paging, which is a big battery drain.