I spent the last few days in Las Vegas at the BlogWorld convention and was certain that my iPhone wouldn't make it into the afternoons (after all, that is the norm considering how frequently I use it). In an effort to conserve its battery, I made one simple switch: I turned "push mail" off.
While this may be obvious to others... that one change kept my phone running without charge the entire day (and with plenty of juice remaining). Yesterday I really put it to the test with regular Safari usage (trying to keep tab on some NFL games that I had a 'vested interest' in).
To extend battery life, most people switch off 3G (which kills the phone's usability) and location awareness. In a bind, I would much rather run on 3G and get email alerts every 15 minutes or on request.