Backlight

The easiest way to save power is decrease brightness of your screen. Difference between the highest and the lowest level is a couple of watts but it also depends on screen's diagonal. Worth of recommendation is simple but useful program called xbacklight using RandR extension. It has text interface but you can use it in applets, other programs or commonly assign to keyboard shortcut.


Research:
Hardware: 15.4'' TFT WXGA Toshiba TruBrite
 
Below you can see comparison of power consumption when backlight brightness is set to 100% and 0%.


Short description of using xbacklight:

COMMAND
       xbacklight [option]
OPTIONS
       -get   Returns the current backlight brightness
       -set [percent]   Sets backlight brightness 
       -dec [percent]   Decrease backlight brightness
       -inc [percent]   Increase backlight brightness
EXAMPLES
       xbacklight -set 75
       xbacklight -inc 10
       xbacklight -dec 5