Best Option: root your phone and install some sort of CPU governor/undervolted kernel.
Things to try: Set brightness to minimum possible, turn wi-fi off, turn auto sync off or to do it only every few hours, turn data off (if not in use).
ALSO: I have an HTC phone, but this may work... type in ##4636## in your dialer and call it. If it works correctly it will bring up a menu. Go into your phone information and change your preferred network type to something else (depending on what your carrier is, I use GSM Auto)