I think you should (if you haven't already) condition the battery. I know some will say you don't have to with Lithium Ion batteries. But in my experience with the N900, after fully charging and fully discharging several times, my battery life has extended. I think what drains the battery a lot depends on what you do with it. Simple websurfing over the cellular network (for me Tmobile USA) takes up little battery juice. Example, surfing websites like reading sports, news, etc. Even sending SMS and a little phone conversations, and a few pics are fine.

But once you go to websites like Youtube and watch videos, the battery starts to drain significantly. Or playing videos in your library.

Are you using your N900 on Youtube a lot or playing videos in your library?