1.) Turn off Push email and notifications.
2.) Turn off 3G and Wifi when you're not using them.
3.) Turn screen brightness to 50%.
By doing this, my phone lasted me 1 day, 6 hours, and ~39 minutes. I am jailbroken.
Make sure you calibrate your battery, etc.
(image was taken as soon as phone turned back on from dying)
(e) added another image for those who may be skeptical
I get that with a 3G. You're impressed with that?