I had the same problem when I upgraded from iOS 6 to iOS 7. The battery life reduced to around 6 hours from a full charge.
I checked the settings and even reinstalled my iCloud backup, but the battery life was still bad.
The only option was to do a clean install - connect to iTunes, restore and set up as a new device. After doing this the battery life went back up to around 12 hours.
Ive done the same thing with iOS 8, not because of battery life, but because it seemed a bit glitchy after the upgrade. I didn't check the battery life before I decided to do the clean install.
Its early days, but the performance seems much smoother after the clean install and, after 2 hours of use, the battery is still showing 91%.
You do lose some data doing a clean install, progress in games etc, so its probably best to only do it as a last resort.