Over at The Origami Project team blog, Ross provides some useful insights to UMPC battery management; the approach is really geared towards Vista machines, but you can certainly apply the concepts to UMPCs running Window XP Tablet Edition 2005. Ross also created a Vista Sidebar gadget to help you get a better read on your battery life; you can download that right here.
Some personal modifications to Ross’s great thoughts: I routinely keep my screen backlight to the lowest setting that I can use. I also take advantage of different power settings between an AC profile and a battery situation. If I know I’m not going to use my Bluetooth keyboard for any length of time, I’ll actually disable the Bluetooth radio in Device Manager. I set my power button to put my device in Standby and I use it often. What tips would you add?