In order to save power, I often turn off my LCD monitor when I away from keyboard for long. Bare in mind, blank screen is not consider turn off the monitor, because it still uses backlight. To do that, first you need to make sure you have ACPI enabled in your kernel.
If this exist, then you can turn off the monitor by running the command line bellow.
xset dpms force off
xset is a util for setting property of X, dpms force off is to turn off the backlight.
What if I want to set it so that it can be done automatically? Check out xscreensaver at advance tab, there are power management where your can set the duration of getting idle it turns off the backlight. Another alternative you can do is to edit your xorg.conf. Check out this gentoo wiki and mini-HowTo for more information.