Do you run apps in the backround? What mediaplayer do you use (if any)? Are you connected to wireless all the time? Is GPS on all the time? How old is your battery? How often do you charge it?

You can try LV settings (low power), this will make your N900 run on 250/600 but lower voltage, so longer battery life (also slower). You should install Auto Disconnect and set it up to disconnect from any network after 5-10mins. Disable BT, disable GPS, and listen to MP3 instead of internet radio. Check out your homescreen widgets and remove CPU-hogs like Facebook. Now you should last 12-14 hours with 3-4 hours of ebook reading while listening to MP3, 1-2 hours of surfing the net (with Opera 11, turbo on), 20-30 mins of calls, and 30mins of AngryBirds. Also go and get a replacement battery, or even better an external charger like the Justmobile Gum Pro, that is good for 3 charges (4400mAh).