One of the most important tips offered by users and experts alike is to regulate the temperature in which the iPhone operates. The lithium-ion (Li-ion) battery of the iPhone functions best in temperatures ranging from 32 to 95 degrees Fahrenheit, or 0 to 35 degrees Celsius. At its extremities, the iPhone can still function between -4 and 113 degrees Fahrenheit (-20 to 45 degrees Celsius) but temperatures beyond these ranges will be detrimental to your iPhone battery. In other words, leaving your iPhone in a freezer or in a hot car is probably not a good idea. Keep it out of direct sunlight as well.
This next tip culled from forums and the like is not so clear-cut. There are some who claim that allowing the iPhone battery to completely discharge more than once a month is not advisable, while others believe that no harm can come of it. What sees to have a consensus is that the iPhone battery must be fully discharged from a full a discharge once a month in order to reset the battery meter.
It is also useful to have an idea of your actual usage as well as your battery life in order that you can more fully understand your options. You can view your usage statistics in the Settings Menu under the General Tab then Usage. You will see Usage, which is the time that the iPhone was in active mode which means you were on a call, watching a video, listening to music, browsing the Internet, and so on. It will also show your Standby time, which is the total amount of time the iPhone has been running from its last full charge, whether in active mode or not. Usage divided by Standby time will give you a percentage of the time you were actively using your iPhone. The higher this percentage, the more often you will need to charge your iPhone, so plan accordingly.
Updating to the latest official version of the software using iTunes may help you with taking care of the iPhone battery, but that's still up for grabs. Some users complain that the update from 3 to 3.1 waked havoc on some applications. However, updates and patches are supposedly to help improve the performance, so maybe that last one was a bit of a fluke. At any rate, before updating your software, check out the forums to see what the feedback is from those who have already updated their software.
And of course, the old standby of using on the applications you need, turning off push, 3G and lower the brightness of the display and so on are still part and parcel of keeping your iPhone battery in good health for a longer period. Just because you've heard it before does not mean it does not work.
Something new, though. Some users have noticed that proxy to a cell tower (thus providing access to a strong signal) extends the battery life. This may be because network searching is brought to a minimum. So try to stay in areas with a good signal. A last and final tip is to use your iPhone regularly, so you massage the kinks out of the system and become familiar with the peculiarities of your particular unit. After all, your phone is a highly personal gadget, and should be treated as such.