The analogy of pretending you are sitting down on a stool is a good one that gets the feeling of sticking your rear end out and bending the knees slightly. I feel that keeping the chin up and out of your chest is a very important aspect of posture that you should check every shot. I know these aspects of the golf set-up may seem boring and may not appear important to shot making, but let me assure they are essential. Most accomplished golfers will tell you that when they are having trouble with their game, they can often trace it back to one of these aspects. I encourage you to get into the habit of checking each Grip, Alignment, Stance and Posture everytime you address the ball, even on the range.
Every good golfer has a pre-shot routine and sticks to it, no matter what.
Like A Pro, a video that outlines a complete fitness program for golfers.