When I got the N900 battery life was terrible!
I put the thing for sale the next day. As I used it I liked it more and more so I played around and read posts here and there and finally found a great solution!

This may or may not work on yours but very well worth a try.

3 things you need to do.

1ST

- Let you battery run DEAD on it's own.
- Charge it up till the phone says it's fully charged.
- Now here's were you need patience. after it's fully charged, plug it up AGAIN. IF it still says fully charged than your good to go. On mine it said fully charged but when I pluged again it continued to charge for almost 2 hours!

NOTE: keep plugging your device again after the phone says it's FULL. The phone eventually will be calibrated with the battery! Don't listen to "the battery is this or that it does not need to be..... Just do it!

2ND

Go to settings,
internet connection,
click on CONNECTIONS TAB,
now edit your WIFI connection,
click next,
click next again,
next again,
now click on ADVANCE TAB,
now click OTHER,
on wi-fi transmission choose 10 mW.
on POWER SAVING choose MAXIMUM.
no click on SAVE.

This will give you very good battery life if you use wifi alot.
Before I could NOT leave wifi on, the battery will be down 50% in 3 hours! Now I use the wifi to stream a 42 minute BONES/FRING tv show and the battery is still shows green all the way to the top. Before I be down 40%!

Just browsing the battery lasts and lasts. I also should point out I have the app that lets you switch to 3G or GSM. I have it set to GSM when I dont need 3G. I will test the battery difference between the 3G and GSM when I get some time.

Try this and let me know how it works out for you.