If the G9 can turn the display off with the disp button (for shooting through the viewfinder) that should be the best option. This turns off all the live view hardware in addition to the backlight, which saves a significant amount of power. Most cameras with an optical viewfinder can do this. Turning off the backlight alone doesn't actually save much.
If not, you can turn the display off after every shot, or simply set the canon sleep interval to something very short. If you are doing thousands of shots, this might not be good for the backlight hardware (if it is CCFL based, I don't know)
Plugging something into the av port will also turn off the display, but again, this doesn't actually save much power. On cameras where the AV port isn't combined with the USB, this can be any non-conductive item that fits, there's a mechanical switch in the plug.