Multitasking on iOS 4 however brilliant is an easy way to drain iPhone battery. Double press the home button and then hold your finger over one of the application icons. Kill any unnecessary applications by tapping on the red icons.
Turning the iPhone 4’s flash off will save plenty of juice.
If you aren’t a massive mobile data user then consider keeping 3G and cellular data switched off until you need them. Although irritating having to switch them back on and off again, this will save you serious amounts of battery.
Screen auto-lock will send the handset to sleep without you needing to remember to tap the on-off switch. Perfect for the more forgetful iPhone user.
Keep Bluetooth off unless you are using it.
Push notifications mean that most applications will run in the background, keep this slider on off and the battery should last a lot longer.
Auto-Brightness will stop you draining juice with an unnecessarily bright screen.
If you are underground or in an area without reception for extended periods of time, consider turning on Airplane Mode. Without any of the phone functions of the handset running the battery has some time to relax.
Don’t use Wi-Fi unless in range of a network you know you can access. While it is switched on the handset is continually searching for available networks which results in extra battery use.