See list below on what Apple is suggesting to extend your iPhone 3G’s battery life.
* Turn off 3G
* Minimize use of location services
* Fetch new data less frequently
* Turn off push mail
* Auto-check fewer email accounts
* Minimize use of third-party applications
* Turn off Wi-Fi
* Turn off Bluetooth
* Use Airplane Mode in low- or no-coverage areas
* Adjust brightness
* Turn off EQ
Bascially, they want you to turn off the damn iPhone.