The biggest wolfers of electricity on your iPhone are its screen and its wireless features. Therefore, you can get substantially longer life from each battery charge by using these features:
Dim the screen. In bright light, the screen brightens (but uses more battery power). In dim light, it darkens.
Note
This works because of an ambient light sensor that's hiding behind the glass above the earpiece. Apple says that it tried having the light sensor active all the time, but it was weird to have the screen constantly dimming and brightening as you used it. So the sensor now samples the ambient light and adjusts the brightness only once—when you unlock the phone after waking it.
You can use this information to your advantage. By covering up the sensor as you unlock the phone, you force it into a low-power, dim screen setting (because the phone believes that it's in a dark room). Or by holding it up to a light as you wake it, you get full brightness. In both cases, you've saved all the taps and navigation it would have taken you to find the manual brightness slider in Settings (Brightness).
Turn off Wi-Fi. Tap Home→Settings→Wi-Fi→On/Off. If you're not in a wireless hot spot anyway, you may as well stop the thing from using its radio.
Or, at the very least, tell the iPhone to stop searching for Wi-Fi networks it can connect to. Wi-Fi has the details.
Turn off the phone, too. In Airplane mode, you shut off both Wi-Fi and the cellular radios, saving the most power of all. Turning Off the Antennas—and Airplane Mode has details.
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