Users who have upgraded from iOS 4 to iOS 5 are reporting a decreased battery life.

Apple will probably fix the issue in the iOS 5.0.1 but for the time being there are a few things you can do to improve the duration of your battery:

Turn off Vibrate
Your iPhone consumes a lot of battery when vibrating. If you are receiving a lot of calls every day, and if you can live without it, turn off the vibration.
Go to Settings > Sounds > Turn off Vibrate

Disable Ping
Ping, used by Apple’s music service, is one of the culprits for a poor battery life and if you don’t use it, you should definitely disable it.
Go to Settings > General > Restrictions > Enable Restrictions > Ping > OFF

Disable Bluetooth and Wi-Fi
To turn off the Bluetooth and Wi-Fi when you don’t need them, only takes a few seconds but can grant you minutes of phone conversation.
Settings > General > Bluetooth > OFF
Settings > Wi-Fi > OFF

Disable iCloud
This is another battery drainer
Go to Settings > General > iCloud and turn everything OFF except for “Find My iPhone” if you have it set up.

Disable Diagnostic and Usage
Navigate to Settings > General > About > Diagnostic and Usage and tap on Don’t Send

Disable Location Services
If you have a lot of applications installed, you need to select disable the ones you don’t want your location to be constantly sent to the servers.
Go to Settings > Location Services > Go through the list of app and disable the services you don’t use, like games and others.

Turn Off Time Zone Adjustment
Those who don’t travel on a regular base won’t need to keep the time adjustment feature on.
Go to Settings > Location Services > System Services > Setting Time Zone > OFF