How to simulate a retina display (HiDPI mode) in Mac OS X 10.8 Mountain Lion on a non-retina display?

Question

How can you simulate a retina display (HiDPI mode) in Mac OS X 10.8 Mountain Lion on a non-retina display?

Answer 1

For those unable to enable HIDPI on rMBP or new MBA, I experienced the same on my rMBP 15" with Air Display. I solved the problem by installing SwitchResX. With the boolean setting enabled as shown in the referenced gist, the HIDPI setting shows up.

Dragging seems a little laggy in Air Display, but otherwise works great.