In Mac OS X v10.6.8 and earlier, wake the computer up by clicking the mouse button or by pressing a key on your external keyboard.
In OS X Lion and later, the external display will change to a blue screen, then will show your desktop.
After your computer's Desktop appears on the external display, close the computer's lid.
With the computer turned on connect the Mac notebook (using the appropriate Apple adapter if necessary) to the appropriate port on the external display or projector and turn the display or projector on.
Connect a USB keyboard and mouse to your computer.
Make sure the computer is plugged in to an outlet using the AC power adapter.
Use these steps to enable closed clamshell (display closed) mode: If you are using a wired keyboard and mouse: Fred posted a very good support page about how to acheive clamshell mode external displays.