Stand behind the ball and picture where you want the ball to end up. Then (depending on the desired ball flight) pick a line on which you'd like to start the ball that makes sense (i.e., if you're trying to fade back to a target, pick an aim line to the left of the target). Now, align oneself as if trying to hit a straight shot along the aim line (completely neutral stance). If trying to hit a draw turn the back foot out more and the front foot perpendicular to the target line; for a fade, do the opposite.
This encourages either more shoulder and body turn on the backswing (draw with back foot flared out) or more shoulder and body turn on the downswing (fade with front foot flared out).
I initially read about this in Hank Haney's book, The Only Golf Lesson You'll Ever Need, but it makes complete sense if you think about it. The cool thing about doing this to encourage a certain ball flight is that it seems to give just the right amount of inside-out or outside-in, providing slight draws and fades as opposed to more extreme hooks and slices that can come from the other methods (like the traditional setup method--link above).
For a straighter shot attempt (encouraging an in-out-in path), flair both feet out slightly.
Flare your right foot to 2 o'clock and position your left foot at 12 o'clock. Now swing the club for real and feel the increase in shoulder turn going back and the lessening of it into your follow-through.
As you do this, make sure you maintain your posture and forward tilt. Straightening up through impact only encourages a slice. This tricks also works if you hook the ball. In this case, position your right foot at 12 o'clock and your left foot at 10 o'clock. This will decrease your turn back and increase your turn through. You'll hit the ball more with your body and less with your hands so they can't take over and shut down the clubface through impact.
To increase your ability to turn back and through the ball, flare your feet between 15 and 30 degrees at address, depending on your flexibility. The less flexible you are, the more a flared back foot will allow you to turn behind the ball and then approach the target line from the inside on the downswing.
The correct amount of flare in your front foot at address makes it easier to rotate your front hip and knee correctly as you swing through the hitting zone. If your swing is too steep, use more flare; if it's too flat, use less.
[From AskMen.com - Golf slices]