Are you using google chrome by any chance? If so, get rid of that POS ressource hog of a browser and use safari instead, watch your battery life increase by 20% or more.
If you read the fine print, the computer lasts "Up to" X hours. It also mentions how it was achieved. Tests are done browsing a website on Safari which has no flash content, refreshing every 30 seconds, with a Pages document open, bluetooth off, Time Machine off, screen at about 66% brightness.