I have an iPhone 4S and to conserve power, I've turned off WiFi, location services, and Bluetooth.
Nonetheless, with my volume of email, I cannot get more than 12 hours of use from an iPhone. I charge it overnight, use it during the day, then begin recharging it as soon as I get home.
QYG has a solution - a thin iPhone 4S protective case that contains a lightweight flat battery which doubles battery life.
Several of my staff members use these and note that the mini-USB port can be used with a standard USB cable to charge the battery.
A lightweight, protective case that doubles the battery life of an iPhone - that's cool.