It's easy.

Go into the settings of the iPhone... turn off 3G data when you're not using it. Turn off location services. Turn off push notifications. Use Wi-Fi whenever available. Turn the brightness down. You'd be surprised how much longer your battery lasts. On turning off the 3G radio alone, my iPhone 3GS went from barely lasting a day to lasting through two days and into a third before I need to charge it.

Also, double-tap the home button and close the apps that are running in the background. That sucks up data and battery too.