Here is an exercise, using two tour sticks, to help understand and practice being in the correct stance every time.
The purpose of the stance in golf is to provide a solid foundation upon which to rotate the body. During the golf swing, the club head can be travelling at anywhere from 60mph to 120mph, depending on the golfer. To be able to control the club head at this speed, a proper stance is vital as it is the foundation to maintain accuracy and consistency and, if correct, helps to prevent injury.
A golf stance is made up of several elements; the correct width of stance, the correct amount of knee flex, the angle of the feet and the overall posture.
Here, the focus of the practice is on getting the width of the stance correct. The width of the stance is important as it is the foundation to the swing. It needs to be solid to provide resistance to coil against in the backswing, but there also needs to be some movement to let the hips turn backwards and then drive forwards through the ball. If in the address position to the golf ball the feet are too close together, the foundation, the legs, will not resist enough when upon rotation in the backswing, losing coil and therefore, power. The knees will have a tendency to bend too much and the feet may rock, which affects balance as the base is too small to support the swing. Imagine swinging, standing on just one leg. Doing this, there is an inability to generate any power and there will be a loss of balance if the ball is hit hard, which compromises ball striking.
On the other hand, if the feet are too far apart at set up, there will be too much resistance in the foundation of the swing. It will be difficult to turn at all, as the knees and hips will not be able to move. This loses power as there will not be enough coil. Also, it becomes very difficult to shift the body weight during the swing, and so instead of turning, the swing tends to become more of a sway. This not only can increase the risk of injury to the back, but can also prevent any drive forwards through the ball to strike the ball correctly.
The correct width to stand to the ball is to have the feet at shoulder width. This provides the ideal amount of solidity to support the swing at great speeds, but provides enough movement in the hips to enable the body to rotate effectively.
To ensure the correct width of the feet in the stance, measure the width of the outside of the shoulders using a golf club, then place the golf club on the floor, noting how wide your shoulders were in relation to the club. Take the two tour sticks and push the sticks into the ground, so that they are pointing vertically upwards at the points where the shoulder width was measured with the club on the floor.
Now feel free to hit golf shots standing with the feet inside the tour sticks, safe in the knowledge that they are consistently the correct shoulder width apart at set up every time.