This might sound crazy but my solution was to use the Wizmo utility at www[dot]grc[dot]com and place a link on the desktop. When I want to switch the screen off I just double click the Wizmo link. When you want the screen back on just waggle the mouse. I don't touch the monitors power button. The only other solution other than switching everything off and back on again is to unplug both power and video (DVI or VGA) to the monitor and then re-plug them in without switching the PC off. Obviously this could be dangerous.