This may seem simple... but have you tried all of the techniques listed to reduce battery such as:
- Reducing screen brightness
- Turning off mobile data (in particular 3G) when not in usage
- Turning off location services
- Killing any apps which could be backgrounding such as Spotify
More here... http://www.apple.com/batteries/iphone.html
If none of them work, and if sepehrg's possible solution doesn't work, take it into an Apple retail store and they may exchange it if there is an evident hardware problem. If you choose to do that, remember to say that you've tried everything listed above and a solution may be much quicker.