With Mac OS X Lion, MacBooks lost the ability to turn off the laptop's screen when you connect to an external monitor. It sucks more than you would think. Here's a simple way to (mostly) fix the problem. Updated with improved Terminal version!
You can still run a single external monitor if you close the lid of your laptop, but that restricts airflow and exposes your display to a lot of heat. Not good.
Instead, go to Displays>Arrangement in System Preferences and change the 'main' display to your external monitor by dragging the taskbar from your laptop's screen to the other screen's icon. Now drag the laptop's screen down a bit, so it's diagonally connected, instead of horizontally. And you're done.
There are more than a few benefits to this. One is that if you're playing a fullscreen game, you won't risk losing your cursor off to the left or right of the screen. Well, you minimise it, at least. If you lose it in the bottom left corner, you're probably going to die horribly before you get it back. But it's much, much better, at least.
Also, if you keep your Dock on the left side of your screen on your laptop, it will be there on your external monitor. That's especially useful if you're, say, dragging an image from your desktop or a folder onto an icon on your Dock, like Photoshop. And if you use any third-party software to get Windows' Snap functionality, you can snap to either half of your screen again.
This won't fix any resource drain from the second screen, but it solves a lot of the headaches associated with the change.
Update: Former Apple Genius and Giz reader Matt Wood wrote to us with a Terminal command to actually shut off the whole monitor:
I used to work at the Genius Bar and got the "how do I turn off my MacBook monitor only?" question a lot.
Here is a terminal command thats fairly simple that will allow you to keep the lid open for adequate cooling but while using the external display only. I've been using this method with my Thunderbolt display and MacBook Air since the beta of Mountain Lion came out and it works perfect. Has all the same characteristics of good old clamshell mode. Be sure to keep this written down someplace in the off chance you need to re-install, do a PRAM reset, or just want to undo it.
To execute in Terminal:
sudo nvram boot-args="iog=0x0"
To undo in Terminal:
sudo nvram -d boot-args
Once you type it into terminal I believe you need to enter your password. I then restart my machine. Now the TRICK is to either restart your machine with the lid already closed (hit restart then slam the lid!) OR turn the machine on for the first time (then quickly slam the lid!) once you are past the login screen you can open the lid.
ALSO: if the machine is asleep, and you simply forget and open the lid, it will wake up both displays. So just remember to wake the machine up with an external bluetooth mouse or keyboard.
This works just like clamshell mode - including all its dumb quirks.
We tested it and it works perfectly. Thanks Matt!