This might sound strange but unless you need the speed of browsing the internet at 3G speeds instead of WiFi you should be able to double your talk time:
Settings > General > Network > Slide the Enable 3G to OFF

By covering up the sensor (just above the earpiece) as you unlock the phone, you force it to a low-power, dim screen-brightness setting (because the phone believes that it's in a dark room). Or by holding it up to a light as you wake it, you get full brightness. In both cases, you.ve saved all the taps and navigation it would have taken you to find the manual brightness slider in Settings.