Tips on battery use from iRex webpage:
http://www.irextechnologies.com/batteries
Also, if you look through posts here and on the iRex forum, you'll probably find more stuff (like power-saving IPDF reader, etc).
p.s. It's impossible to find the link above just from the iRex page (I had to go and copy it from the thread I knew had it). That makes no sense, having a page with tips and no way to find it for the users who are not "in the know"/on the forum.