Turn off Consilient Push when out of coverage
Turn off non-essential features when idle
Lower screen brightness
Turn off animated screensavers/wallpapers
Turn off vibrate mode and ring tones if not needed, turn down volume as well
If you're browsing the Web on a GPRS connection, make sure you set the GPRS connection to "only as needed" so that it turns itself off when you don't need it.
Purchase an extended battery