This is serious stuff, considering each battery only has so much life and can only be replaced by mailing it in to the Apple mothership. (Insert pained expression here!)
I found this tip on iPhone Atlas, which has a treasure trove of useful tips and dirt on everything you could possibly want to know about the iPhone but were afraid to ask.
Apparantly, if your battery is draining faster than you think it should; it may be your e-mail settings. Everytime it checks for new e-mail, it dies a little more.
For example, iPhone Atlas points out that if you use Yahoo! e-mail; it may be set to "push e-mail" which notifies the user every time something new hits the inbox. Check out your advanced settings, if you think this applies to you.
For other email clients, play around with the settings and see how often it's supposed to check for new email and change it to longer intervals.