Really the only way to increase to increase the battery is to do the fallowing:
--Turn off wi-fi
--set the back light to turn off after 5 min
--carry extra battery
You can also dim the backlight to a very low level. That definitely helps a lot.
You do realize the battery isn't designed to be user-replaced, don't you?
If you carry an extra battery you better also carry a tool set to take the computer apart to replace it! If you're field-replacing the battery you would also cause a lot of other problems such as system clock resetting, etc.
In the end it'd probably be more convenient to carry some kind of solar panel or gas generator to recharge the existing battery.