I bought the 3G iPhone, just like a bunch of people I know. I love it, but I do wish the battery life was longer. It goes down pretty fast, even though I've already done most of the tips that Apple has. I know many of you are looking for this info, so here it... straight from Apple itself!
Minimize use of location services: Applications that actively use location services such as Maps may reduce battery life. To disable location services, go to Settings > General > Location Services or use location services only when needed.
Fetch new data less frequently: Applications such as Mail can be set to fetch data wirelessly at specific intervals. The more frequently email or other data is fetched, the quicker your battery may drain. To fetch new data manually, from the Home screen choose Settings > Fetch New Data and tap Manually. To increase the fetch interval, go to Settings > Fetch New Data and tap Hourly. Note that this is a global setting and applies to all applications that do not support push services.
Turn off push mail: If you have a push mail account such as Yahoo!, MobileMe or Microsoft Exchange, turn off push mail when you don’t need it. Go to Settings > Fetch New Data and set Push to Off. Messages sent to your push email accounts will now be received on your phone based on the global Fetch setting rather than as they arrive.
Auto-check fewer email accounts: You can save power by checking fewer email accounts. This can be accomplished by turning off an email account or by deleting it. To turn off an account, go to Settings > Mail, Contacts, Calendars, choose an email account, and set Account to Off. To remove an account, go to Settings > Mail, Contacts, Calendars, choose an email account, and tap Delete Account.
Minimize use of third-party applications: Excessive use of applications such as games that prevent the screen from dimming or shutting off or applications that use location services can reduce battery life.
Turn off Wi-Fi: If you rarely use Wi-Fi, you can turn it off to save power. Go to Settings > Wi-Fi and set Wi-Fi to Off. Note that if you frequently use your iPhone to browse the web, battery life may be improved by using Wi-Fi instead of cellular data networks.
Turn off Bluetooth: If you rarely use a Bluetooth headset or car kit, you can turn off Bluetooth to save power. Go to Settings > General > Bluetooth and set Bluetooth to Off.
Use Airplane Mode in low- or no-coverage areas: Because your iPhone always tries to maintain a connection with the cellular network, it may use more power in low- or no-coverage areas. Turning on Airplane Mode can increase battery life in these situations; however, you will be unable to make or receive calls. To turn on Airplane Mode, go to Settings and set Airplane Mode to On.
Adjust brightness: Dimming the screen is another way to extend battery life. Go to Settings > Brightness and drag the slider to the left to lower the default screen brightness. In addition, turning on Auto-Brightness allows the screen to adjust its brightness based on current lighting conditions. Go to Settings > Brightness and set Auto-Brightness to On.
Turn off EQ: Applying an equalizer setting to song playback on your iPhone can decrease battery life. To turn EQ off, go to Settings > iPod > EQ and tap Off. Note that if you’ve added EQ to songs directly in iTunes, you’ll need to set EQ on iPhone to Flat in order to have the same effect as Off because iPhone keeps your iTunes settings intact. Go to Settings > iPod > EQ and tap Flat.
Turn off 3G: Using 3G cellular networks loads data faster, but may also decrease battery life, especially in areas with limited 3G coverage. To disable 3G, from the Home screen choose Settings > General > Network and set Enable 3G to Off. You will still be able to make and receive calls and access cellular data networks via EDGE or GPRS where available.
The funniest one is to "turn off the 3G". Why would I do that? I just bought it because of the 3G!!!