This is like asking why does my car run out of gas?
(not being sarcastic, just we need more information to help)
What carrier do you have?
What are your usage patterns?
Do you travel around cities for your job?
Do you stay in one spot all day?
We have to know what you do to advise on the best-case for battery life.
The general steps are:
Turn off 3G when you are in service areas with low signal/use wifi more.
Turn off Push/Fetch for mail
Lower the brightness