Your feet should start out shoulder-width apart, but adjust depending on whether you're playing woods/long irons, middle irons or short irons.
Photo by Kelly Lamanna; used with permission
The feet should be shoulder width (outside of the shoulders to the inside of the heels) for the middle irons. The short iron stance will be two inches narrower and the stance for long irons and woods should be two inches wider. The target-side foot should be flared toward the target from 20 to 40 degrees to allow the body to rotate toward the target on the downswing. The back foot should be square (90 degrees to the target line) to slightly open to create the proper hip turn on the back swing. Your flexibility and body rotation speed determine the proper foot placement.