One of the most frequent complaints about the iPhone is how fast the popular smartphone sucks the energy right out of the battery. And this is compounded by having the battery sealed within the phone, so carrying around a replacement won't work. But there are many things you can do to save iPhone battery life simply by going into the iPhone settings and adjusting a few things.
Apple says to:
Optimize Your Settings
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Minimize use of location services: Applications that actively use location services such as Maps may reduce battery life. To disable location services, go to Settings > General > Location Services or use location services only when needed.
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Turn off push notifications: Some applications from the App Store use the Apple Push Notification Service to alert you of new data. Those applications that extensively rely on push notifications (such as instant messaging applications) may impact battery life. To disable push notifications, go to Settings > Notifications and set Notifications to Off. Note that this does not prevent new data from being received when the application is opened. Also, the Notifications setting will not be visible if you do not have any applications installed that support push notifications.
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Fetch new data less frequently: Applications such as Mail can be set to fetch data wirelessly at specific intervals. The more frequently email or other data is fetched, the quicker your battery may drain. To fetch new data manually, from the Home screen choose Settings > Mail, Contacts, Calendars > Fetch New Data and tap Manually. To increase the fetch interval, go to Settings > Mail, Contacts, Calendars > Fetch New Data and tap Hourly. Note that this is a global setting and applies to all applications that do not support push services.
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Turn off push mail: If you have a push mail account such as Yahoo!, MobileMe or Microsoft Exchange, turn off push when you don’t need it. Go to Settings > Mail, Contacts, Calendars > Fetch New Data and set Push to Off. Messages sent to your push email accounts will now be received on your phone based on the global Fetch setting rather than as they arrive. If the global Fetch setting is set to Manually, you will not be able to locate your iPhone using the MobileMe Find My iPhone feature.
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Auto-check fewer email accounts: You can save power by checking fewer email accounts. This can be accomplished by turning off an email account or by deleting it. To turn off an account, go to Settings > Mail, Contacts, Calendars, choose an email account, and set Account to Off. To remove an account, go to Settings > Mail, Contacts, Calendars, choose an email account, and tap Delete Account.
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Minimize use of third-party applications: Excessive use of applications such as games that prevent the screen from dimming or shutting off or applications that use location services can reduce battery life.
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Turn off Wi-Fi: If you rarely use Wi-Fi, you can turn it off to save power. Go to Settings > Wi-Fi and set Wi-Fi to Off. Note that if you frequently use your iPhone to browse the web, battery life may be improved by using Wi-Fi instead of cellular data networks.
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Turn off Bluetooth: If you rarely use a Bluetooth headset or car kit, you can turn off Bluetooth to save power. Go to Settings > General > Bluetooth and set Bluetooth to Off.
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Use Airplane Mode in low- or no-coverage areas: Because your iPhone always tries to maintain a connection with the cellular network, it may use more power in low- or no-coverage areas. Turning on Airplane Mode can increase battery life in these situations; however, you will be unable to make or receive calls. To turn on Airplane Mode, go to Settings and set Airplane Mode to On.
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Adjust brightness: Dimming the screen is another way to extend battery life. Go to Settings > Brightness and drag the slider to the left to lower the default screen brightness. In addition, turning on Auto-Brightness allows the screen to adjust its brightness based on current lighting conditions. Go to Settings > Brightness and set Auto-Brightness to On.
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Turn off EQ: Applying an equalizer setting to song playback on your iPhone can decrease battery life. To turn EQ off, go to Settings > iPod > EQ and tap Off. Note that if you’ve added EQ to songs directly in iTunes, you’ll need to set EQ on iPhone to Flat in order to have the same effect as Off because iPhone keeps your iTunes settings intact. Go to Settings > iPod > EQ and tap Flat.
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Turn off 3G: Using 3G cellular networks loads data faster, but may also decrease battery life, especially in areas with limited 3G coverage. To disable 3G, from the Home screen choose Settings > General > Network and set Enable 3G to Off. You will still be able to make and receive calls and access cellular data networks via EDGE or GPRS where available.
And CNET says this:
Reset iPhone
In addition to resolving signal strength problems, resetting an iPhone can fix a faulty battery indicator and end applications that are improperly consuming the battery. Hold down the home and sleep buttons simultaneously until you see the white Apple logo indicating that your iPhone has restarted.
Restore with original settings
You can restore your iPhone by launching iTunes and selecting the Restore option on your iPhone's Summary page. This process will erase your iPhone's software and data, including all settings. You can restore them later via iTunes.
Drain the battery completely
Instead of charging it midway through a battery drain, add more juice only after the battery dies completely. A full drain may reset circuitry that prevents a full charge from occurring, despite indication of a full charge.
Force quit applications
If an application quits unexpectedly and you begin to notice poor battery life, relaunch the application, then force quit it by following the directions in this post. This can eliminate problematic or hung processes that might be draining the battery.
Lock your iPhone
Make sure that your iPhone goes to sleep automatically when not in use. You always can press the Sleep/Wake button, but even better is the Auto-Lock feature in the General section of the Settings menu.
Auto lock screen
(Credit: David Martin)
Applications
Apps that keep the display from dimming or shutting off and that prevent your iPhone from sleeping can reduce battery life. This is particularly true for apps that use location services and push notifications. You can turn off Location Services in Settings > General menu. You'll have to deactivate Push Notifications for each app individually, also in the Setting menu.
Push notifications for e-mail
The iPhone 3G's Push functionality for Exchange and Mobile Me accounts also can destroy battery life. Turn this option off in Settings > Mail, Contacts, Calendars > Fetch New Data before restarting your iPhone. Messages sent to your push e-mail accounts will now appear on your phone based on the global Fetch setting (every 15 minutes, every 30 minutes, hourly, or manually) rather than as they arrive. The manual Fetch setting will preserve the most battery life.
Push and Fetch
(Credit: David Martin)
Checking fewer e-mail accounts
Deleting e-mail accounts or turning them off can preserve battery life as well. You can remove an e-mail account in Settings > Mail, Contacts, Calendars, tap to choose an e-mail account, and tap Delete Account. To turn an account off but leave it on the phone, go to Settings > Mail, Contacts, Calendars, tap to choose an e-mail account, and set Account to Off.
Toggle e-mail account on/off
(Credit: David Martin)
Delete e-mail account
(Credit: David Martin)
Airplane mode
Even it you're not flying, use the airplane mode while listening to music, watching a video, working in your favorite productivity app, or whenever you don't want to be disturbed. Also, since constantly seeking a cellular connection also drains the iPhone's battery, the airplane mode is useful if you're in an area with poor cellular coverage.
Airplane mode
(Credit: David Martin)
Reset network settings
Restarting your phone will delete any stored Wi-Fi passwords, DNS settings, and more. And in some cases it also can resolve signal strength issues. Find it in Settings > General > Reset > Reset Network Settings.
Reset network settings
(Credit: David Martin)
Turn off Wi-Fi and/or Bluetooth
Be sure to turn off these features when not in use. Go to Settings > Wi-Fi and set Wi-Fi to Off. Go to Settings > General > Bluetooth and set Bluetooth to Off.
Toggle Wi-Fi
(Credit: David Martin)
Toggle Bluetooth
(Credit: David Martin)
Turn off 3G
The iPhone's 3G radio consumes a lot of juice so turning if off will save power. Go to Settings > General > Network and toggle Enable 3G to Off. You'll be able to make and receive calls and access some data while 3G is off, though your iPhone will run on the slower EDGE or GPRS 2G network.
Enable/Disable 3G
(Credit: David Martin)
Adjust brightness
Dimming the screen definitely can extend battery life. Go to Settings > Brightness and drag the slider left to lower brightness or right to increase brightness until you reach your desired settings. Turning off the Auto-Brightness settings also will help.
Brightness settings
(Credit: David Martin)
Turn off the equalizer
Just like on the iPod, this feature can reduce battery life. Go to Settings > iPod > EQ and select Off. Yet, since EQ settings in iTunes migrate to the iPhone, you'll have to override those settings and set the iPhone to Flat. Go to Settings > iPod > EQ and select Flat.
iPod EQ options
(Credit: David Martin)
iPod EQ settings
(Credit: David Martin)
You can also carry around a spare battery, like our GroovePower Now
