There's an old tip here on how to use your MacBook with the main display off -- i.e. just using an external monitor. However, it requires that you have USB things to plug in, and it's a pain. This way is much easier.
With your display on both main and external monitors, start closing the laptop lid, but don't close it all the way. Get it so that there's a finger's depth between the lid and the palm rests. The MacBook should go to sleep, thinking you've shut it. Now reach underneath the lid and start pressing some keys -- the right arrow key is easiest to reach. You may have to bend the lid up a bit, but as long as it doesn't turn at the back, the MacBook won't think you're trying to open it again.
After pressing the keys a few times, the computer will wake from sleep and will reconfigure the display so that the main display is off. Now you can open the MacBook lid as far as you like -- I have it at about 30 degrees so I can type, but it doesn't get in the way of the monitor. And there you go! It might not work the first time, but it does work. I can now have my MacBook's screen off without any pesky USB switching.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070504003442646