Turn off your screen saver and have the computer shut down the screen when you're not using it.

Did you know that the phrase "Screen Saver" is now a misnomer? Modern LCD screens aren't damaged by showing one image for a long time (the situation screen savers were originally created to avoid), but screen savers use up the lifespan of the bulb that lights the screen. The display also consumes just as much power when it's showing a screen saver as when you're working. Have your computer shut down the display entirely when you're not using it rather than running a screen saver. You'll consume less electricity and reduce the need for expensive bulb replacements.

In Windows 7, click the Windows logo button and type turn off display in the search box. The top result will be Choose when to turn off display. Click on it, and you can then set how long the computer should wait before turning off the display.