What apps do you have running in the background - i.e. Facebook, GPS/Maps, etc?
Keep in mind that you do not want to let the battery get below 20% on any sort of regular basis. Doing so can damage the Lithium battery. That might be the problem as well. Had a similar issue with my BB 9000. I keep the 9800 topped off when I am in the office.
This might help: Maximum Battery Life - BlackBerryFAQ