I've probably been asked more about this piece of gear than any other that I own so I figured I'd start by posting it here.
It's pretty simple. You simply thread this thing below through your harness, screw in one of the ball attachments into your camera and you can quickly and securely holster it with one hand.
Here is a picture of someone with the whole setup on their belt.
I've had it for a year now and I love it. It's available for $50.
More info on their website: http://spiderholster.com/black-widow/