Apply all or some of these tips and your battery life should get better. Note that this article is inspired by Apples website.
Keep your iPhone off the heat. It works best from 32° to 95° F
Update your iPhone with the latest firmware (unless youre a jailbreaker)
Turn off wifi if youre not using it (Settings > Wi-Fi and set Wi-Fi to Off)
Turn off 3G if you dont need it (Settings > General > Network and set Enable 3G to Off)
Turn off song EQualizer (Settings > iPod > EQ and tap Off)
Turn off Bluetooth (Settings > General > Bluetooth and set Bluetooth to Off)
Turn off Push Notifications (Settings > Notifications and set Notifications to Off)
Turn off Location Services (Settings > General > and Location Services to Off)
Turn off Push Email (Settings > Mail, Contacts, Calendars > Fetch New Data and set Push to Off)
Fetch new data manually (Settings > Mail, Contacts, Calendars > Fetch New Data and tap Manually)
Dont check too many email accounts (Settings > Mail, Contacts, Calendars, choose an email account, and set Account to Off)
Auto-adjust Brightness (Settings > Brightness and set Auto-Brightness to On)
Dont play too much games
Go through a full charge cycle each month (charge the battery to 100% and completely run it down)
Set Auto-Lock for the screen to 1 minute (Settings > General > Auto-Lock > 1 minute)
Turn off Vibrate (Settings > Sounds and set Vibrate to off)