The iPhone is like a little computer. There are loads of programs and processes that drastically drains the battery. Here are some tips on how you can extend the battery life.

(Tips for jailbroken iPhone users will be marked with jb)

1)      Location Services: Programs that uses location services, or in general the GPS will reduce the battery. Once turned off the phone will not use power to gather satellites or connect to the mobile broadband to gather your location.
To deactivate press on Settings > General >Location Services, and turn it off from there.
(jb) You can also turn it off from SB Settings, taken you have added a toggle for it there.

2)      Turn off the Wi-Fi: When you are not using wireless network, it would be vice to turn off the wi-fi, as it uses the battery to search for linkups.
To turn off this function, enter Settings >Wi-Fi and turn it off there.
(jb) This can also be done from SB Settings and Boss Prefs.

3)      Turn off Bluetooth: As Wi-Fi, this function also drains the battery.
To deactivate Bluetooth enter Settings > General > Bluetooth and slide it off there.
(jb) You can also turn it off from BossPrefs and SB Settings.

4)      Use Air plane mode: When you find yourself searching for connection, the iPhone will use a lot of power to connect and maintain connection. If you need to save your battery, and find yourself not needing to call, receive or use your iPhone’s connection, then turn on the Air Plane mode.
To do this, enter settings and turn on Air Plane mode there.
(jb) This can also be done through BossPrefs or SB Settings.

5)      Adjust Brightness: If you have your screen fully illuminated, the battery will quickly drain due to the large screen.
To save power, reduce the brightness by entering Settings > Brightness, and slide it down. You can also activate auto light, the screen will then adjust according to the brightness around.
(jb) You can adjust brightness in SB Settings.

6)      Turn off Push-Notifications: Some programs from the App Store uses Pust Notification to tell you about newly obtained data. These are programs like mail, instant messaging and so on. Push Notification will reduce your battery.
To deactivate, enter Settings > Notifications and slide it to off.

7)      Turn off Push-E-mail: Id you do not use this setting, turn it off by entering Settings > Email, Contacts, Calender > FetchNew Data and slide off Push Notifications there.

8)      If you use the Push-Mail function on your iPhone, you can have it fetch new emails in given time intervals. You can choose yourself how often you wish your phone to fetch new data.
Enter settings > E-mail, Contacts, Calender > Fetch New Data and choose “Every hour”.

9)      Turn off 3G: Data transfer will go much faster when you use the 3G (in my experience, about 200kbps, not bits but bytes). When you do not need 3G, turn it off as it will drain your battery dramatically faster when activated.
To deactivate enter Settings > General > Network, and turn off 3G from there.
(jb) You can also do this in BossPrefs or SB Settings.

10)   Turn off the Equalizer: If you turn off this function, the battery will last longer as it is a process that runs live with the iPod app. Remember; If you have used equalizer settings on songs from iTunes, you must put your iPhone on “flat”.
To turn off equalizer, enter Settings >iPod > EQ, and click off.
To set EQ on “Flat” enter Settings > iPod > EQ, and click “Flat”.

11)   Lock the iPhone: When you are not using your phone, put it on lock mode so the screen goes black. You can put on a short time interval on how long it will take until the screen goes black and locks, for example a minute.
To decide auto lock interval, enter Settings > General >Autolock and choose a short interval from there. Or you can just lock the screen yourself when you are done working on it by pressing the power button once.

12)   Use your iphone!: It is important to use your battery. Sometimes it would be vice to drain the battery, and then charge it to 100 %, drain it, and then recharge it again. This will allow the battery to work better for a longer time.

13)   Avoid extreme temperatures. The iPhone works best under normal temperatures like 22 °C. But, it will also work fine in the temperatures between 0 and 35 °C. It can be kept -20 og 45 °C, but avoid it.

14)   Software: Make sure you always have the newest of iPhone firmwares.

Good luck!