I don't jb, but these are the standard tips to save on battery life that you can find on just about any iPhone 4 battery saving search. I follow all except for 3 and 10 and my battery only goes down about 20-25% with minimal use while I am at work.
1. Dim the Screen brightness – Below halfway saves a significant amount of battery.
2. WiFi OFF, Scanning OFF – If you are not using WiFi turn it off. Also turn WiFi scanning off if you don’t need to search for connections.
3. Toggle off 3G until needed – If you are in an area with no 3G coverage, turn it off.
4. Set the Auto-Lock period to 1 minute – Change this under Settings > General > Auto-lock. This will prevent you from leaving your iPhone on for longer than necessary.
5. Set Push to Off, and Set Fetch Time – Push will “push” your emails to you rather than you having to manually check when you receive an email. This saves battery, so if there is no problem manually checking your mail, turn this off. Also turn Fetch off if you don’t need it, Fetch will synchronizecalendar, contacts and emails.
6. Turn off “Location Services” – Some apps may require the use of Location Services, but If it is not important to you, switch it off.
7. Turn off Bluetooth – Settings > General > Bluetooth > Off.
8. Turn off the music equalizer (EQ) – While this may show an improvement in the sound quality of your music, it also reduces battery life. If you do not care for the EQ much, switch it off. Settings>ipod>EQ>Off.
9. Let the battery completely drain and then fully charge it – Apple recommends that this cycle is performed at least once per month.
10. Use Airplane mode more often – When in low or no coverage areas, switch Airplane mode on to conserve battery. Also when you don’t need the phone for calls etc., switch it on.