As a new owner of a 16gb iPhone 3GS I have had to learn how to deal with the dismal battery life and no way of exchanging batteries mid-day like I did with my Samsung Omnia.  None of the information I am posting is new but suggestions I have found from other blogs and forums from more experienced users.  I just thought if you are frustrated by poor battery life, a battery that lasts less than a day, then you might find a few tips that are new to you.

• Turn off settings not being used. Including
• Wi-Fi – Located under Settings. If you are not connected to a Wi-Fi network there is no reason to have it on. 
• Notifications – Located under Settings. A lot of new applications use push notifications on the iPhone 3GS.  These applications search periodically to see if there is new information to push to your phone eating up battery life.  You can either turn off Notifications completely, control alerts for each application under Settings –> Notifications, and/or make sure that when you install applications to have alerts from only those important to you.  I personally control alerts from under settings as well as have become more selective of what applications I want to receive alerts from…it gets pretty annoying to constantly be receiving alerts on my phone!
• Bluetooth – Located under Settings –> General.  Again no reason to have it on unless you are using a Bluetooth device.
• Location Services – Located under Settings –> General.  This service is another battery drainer.  More and more applications include using your location to determine data to display.  If you are not interested in location-based programs and are not using GPS then you can turn this service off.  I personally just turn it on when I need it.
• 3G – Located under Settings –> General –> Network.  Receiving data over 3G is much faster than AT&T Edge network (or whoever you are using) but if you are only using your phone to make and receive calls then you might as well turn off 3G.  This is especially true if you are in a limited 3G coverage area.  Once you start using applications that access the Internet or Safari then turn 3G back on.  I turn 3G off when I am out and not using anything accessing the Internet except my email which I don’t really need faster access.
• Fetch New Data – Located under Settings –> Mail, Contacts, Calendar.  If you have the setting Push on then when new data arrives it will be pushed to your phone.  However, you can turn this off and fetch data, either at a specified interval or manually.  I have mine set to push because I do count on being notified when new email arrives.
• Brightness – Located under Settings.  I’ve read various opinions about whether you should have Auto Brightness on or off.  Some say they notice no difference in battery life if set to Auto Brightness while others swear by lowering the Brightness down under 50% saves them battery life.  I have tried setting mine manually and didn’t like it because if I went outside then I wasn’t able to read the screen well (just like my Omnia) and would have to set it brighter.  Also, I didn’t notice much difference in battery drain while using my own settings than when set to Auto Brightness.  Rule of thumb though is the brighter the screen, the faster the battery drain.
• Drain battery completely at least once a month from 100%.  Because the iPhone battery is a lithium battery it is important to have it go through one full charging cycle at least once a month.  I try to do it once a week or so because I did find that my battery life improved from when it was new to when I did a completely full charge and discharge.  My battery life got longer.  Excellent!
• Third Party Applications.  Applications that don’t turn the screen off or are constantly relying on location services like GPS will drain your battery quicker.  Be mindful of these applications and limit their use when you need your phone to last a full day.
• Talk Less!  Yep.  If your longwinded friend calls you on a day when you are away from being able to charge it then you might want to let that call go to voicemail and call them at a later time.  Figure you will get about 5 hours on 3G and 12 hours on Edge (showing that talking on Edge network is advantageous).

Now, if you want to use all the services you can and still have a battery that lasts all day you can look at external batteries and battery cases.  I decided that I wanted that option and bought a Mophie Juice Pack Air Case and Rechargeable Battery and I love it!  Be warned however that any battery case is going to add bulk to the iPhone and an external battery will hang off the end of the phone.

Are there ways that you have found to make your battery last longer?  How long does it last?