Base on my personal experience and advice from Apple, here are the 10 tips that can help you make the most out of your iPhone battery:

1. Enable auto lock or Lock your iPhone
When you’re not using the iPhone, make sure it’s locked and put into sleep mode by pressing the Sleep/Wake button. This can preserve battery. If you do not want to lock it manually, you can go to Settings -> General -> Auto-Lock and set the interval (usually it’s set to 1 minute) to auto-lock the iPhone.

2. Adjust brightness
A bright screen also consumes more juice than the dimmer. So, you can set a lower brightness level to extend battery life. You can go to Settings -> Brightness and drag the slider to the left to lower the brightness level. Furthermore, you can turn on Auto-brightness and allows iPhone to adjust the brightness of the screen based on current lighting conditions.

3. Disable 3G
Like Wifi, using 3G cellular network also consumes lots of power. When you’re not surfing the web or in an area that 3G is not supported, switch it off. You can still make and receive calls even 3G service is disabled.

To turn off 3G, simply go to Settings -> General -> Network and set Enable 3G to OFF.

iPhone 3G/3GS eats up battery life and by turning it off when you do not need, it can greatly improve your battery. To easily toggle 3G, you may consider the use of SBSettings.

4. Disable Wifi when it’s not used
Wifi is one of the features that consumes the most of the battery. With Wifi enabled, even when there is not Wifi network around you, iPhone will still consume battery. So, I used to turn it on only when I need to connect to a Wifi network. So, when you’re not using Wifi, switch it off to save battery.

To turn off Wifi, simply go to Settings -> Wi-Fi and set the Wi-Fi option to OFF.

5. Turn off bluetooth

Same as Wifi, if you’re not using bluetooth with your Bluetooth handset or Internet tethering, switch it off to save power. To turn off bluetooth, you can go to Settings -> General -> Bluetooth and set the Bluetooth option to OFF.

6. Disable Location service
The regular use of location service (such as Google Maps) will also impact the battery life. If you do not need to use location service, go to Settings -> General and set “Location Services” to OFF.

7. Disable Push Notification
Push notification is a new feature introduced in iPhone 3.0. Applications (particular for instant messaging app) make use of this notification service to inform you about new update, even the app has been closed. The extensive use of push notification by applications will definitely impact the battery life, as the app keeps polling for any update. So, if you do not need push notification, simply turn it off and you’ll see a great improvement in battery life.

To disable push notification, go to Settings -> Notification and set “Notifications” to “Off”.

8. Turn Off Push Email and Retrieve it Manually

You may love the push email feature offered by Yahoo, MobileMe or MS Exchange but at the time, this feature decreases your battery life. Also, how often do you instruct iPhone to auto-fetch your email? Every 15 min? Here is the rule: the shorter the time interval, the shorter is your battery life.

If you’re not busy businessman, probably you can consider to turn off the push feature and you can even fetch email manually (i.e. emails are fetched only when you launch Mail application). This can greatly improve the battery life.

To turn off Push email, go to Settings -> Mail, Contacts, Calendars -> Fetch New Data and set “Push” to OFF. Also, you can tap “Manually” to disable the auto-fetch.

9. Drain the battery completely once a month

Similar to other electronic devices, the iPhone utilizes a lithium-based battery. For proper use of lithium-based battery, try to completely run down the battery before a re-charge. Normally, we charge the iPhone battery before the 20% warning. And I know it’s impossible to do complete discharge every day. So, I suggest you can drain the battery completely at least once a month.

10. Turn off Equalizer
Do you use Equalizer when playing back song? According to Apple, this feature can also decrease your battery life. To make the most out of your iPhone battery, you may consider to turn it off.

Simply go to Settings -> iPod -> EQ and tap OFF to disable equalizer.

11. Quit An iPhone Application
You can consider to completely quit the iPhone application when they’re not used. This can minimize the processes running background and may save battery. You can refer to my post about “How to Force Quit an iPhone application” for the how-to.