ahmad: I always get crap batteries in my iPhones! Not fair!!
As a final resort when I have some time I might backup all my data and then start fresh as a "new phone".
Today when I hit 20% I had:
Usage: 4 hours 14 minutes
Standby: 10 hours 1 minute
I would recommend doing that I got about 2x more life from my iPod touch after doing that. Made it faster aswell