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Be aware of where you put your iPhone
Almost above all else, keep your iPhone and its internal battery within the operating range, so that it doesn't get too hot or too cold. An iPhone will generate heat depending on what it is being used for, so keeping it at a level temperature is important.
The warmer your iPhone gets, the faster the battery will deplete. Sometimes it's better to keep your iPhone out of your pocket and carry it in your bag — carefully, to ensure the screen or case doesn't get knocked about or damaged — or even clipped to your belt. Some cases will also result in the device getting warm, so choose your accessories and cases carefully.
Adjust human behavior: don't check your phone as much!
While it is often tempting to pick up your phone and check to see if there are any messages, notifications, or missed calls, one of the easiest ways of keeping your battery ticking over is to stop picking it up every few minutes. Putting your phone to "sleep" and "waking" it up will drain battery life.
Cheat! Buy an external battery
If all else fails and battery life continues to be a problem, consider an external battery pack, or carry around a USB charger with you. Some external batteries will extend the battery life of your iPhone for twice the ordinary length, if not longer. There are some wrap-around cases that clip to your phone and can be activated with a switch, though these will make your iPhone thicker in size and heavier to carry. Others plug into the charging port at the bottom to give your iPhone additional juice.