I don't have a 5DmkII, but I do have a 7D (which uses the same battery).
My experience with the 7D (and with the 350D before that) - is to maximise battery time, is to:
- turn off the 'post photo display' (which shows the photo for a period of time after a shot is made)
- or at least minimise the time (eg 2 seconds)
- don't use the LCD menu for camera settings, but instead the dials / buttons directly
- don't use live preview
- keep battery warm before using in cold weather (eg in a pocket close to one's body)
- don't use lots of AI Focus or Servo Focus unless necessary
- use AF button - or set the * button for focus (so that the camera only focusses when you need to, not on every shot)
- turn off IS when not needed
- use flash only when you really have to
- turn screen brightness down
- set the sensor cleaning OFF (that can be set in the 7D on one of the custom menus)
- have camera set to a 1 or 2 minute 'snooze' period (and it will turn back on at the press of a button)
- ensure that when you put the camera in a bag, that camera is turned off, so no button is inadvertently pressed
I get a lot of shots that way per battery (well over 1200 with the 7D - even with use of some flash and screen, and about 400 on the 350D, that's with older batteries... they do 'loose their holding capacity' over time). I've often being on camping trips and multi-day bushwalks (Australian for 'hikes') - so that's important to me!
Hope my post is helpful! :-)
Paul