If you think you're suffering from unusually short battery life, try the following procedures:
Turn off Push The iPhone 3G's Push functionality for Exchange and Mobile Me accounts can destroy battery life. Turn this option off in "Settings" and compare your new battery life to the old.
Force-quit applications If an application quits unexpectedly and you notice poor battery life afterward, relaunch the application then force-quit it by holding down the home button for several seconds. This can eliminate problematic or hung processes that might be draining the battery.
Reset iPhone In some cases, simply resetting the iPhone can resolve signal strength issues. Hold down the home and sleep buttons simultaneously until you see the white Apple logo, indicating that your iPhone has restarted.
This procedure can right a faulty battery indicator and eliminate applications that have hung or are otherwise improperly chewing battery.
Reset network settings Tap Settings, tap General, tap Reset, tap Reset Network Settings. This will cause your iPhone to restart, and will delete any stored Wi-Fi passwords as well as DNS settings and more. It can resolve signal strength issues in some cases.
Bad apps to blame? Anecdotal evidence suggests that apps incompatible with iOS 4.0 or otherwise problematic under the new release can cause battery drain issues. Backup your iPhone in iTunes, then delete apps from the device in groups (you can always sync them back from iTunes later). After each group, restart your iPhone and check for the battery drain problem. Via this method, you can isolate the problematic app and remove it permanently.
Also, make sure all apps are up to date; developers have released a flurry of iOS 4.0-compatible app editions in the past few days.
Lastly, iPhone battery draining fast all of a sudden could mean that you need to handle the device a bit carefully, and take proper care of your iPhone. One of the important aspect of iPhone care is to recharge the battery before it drains completely. This means that, you should not allow the battery to drain completely before charging. Next, you can consider calibrating on your battery to enhance its performance. Calibrating is quite simple and easy, as all you have to do is to charge the battery fully and then use it as usual till the battery has drained completely. At this point, the device will power itself off, after which you again charge the battery fully. However, calibrating should not be done too frequently. Calibrating once in every few months is sufficient to improve the battery life of your iPhone. http://gigaom.com/apple/iphone-3-0-batte... good luck.