How nokia 8800 and outlook application usage last?

Hi guys!
I'm a big fan of Nokia and the time I found this phone I immediately buy one. The specifications are really great.
It's function do more things compare to other Nokia series and even other phone manufacturer is awesome.
I'm using this nokia 8800 and outlook windows application for years already since I started working on a law firm.
My only problem is the battery life. I have to charge it several times within a day.
I don't know if it is because of the application or the battery life is really poor.
Can someone tell me how to strengthen the battery life of my phone?
Suggestions are highly appreciated.
