There are tons of techniques and principles employed by professional golfers and trainers to enhance their golf swing and ultimately their game. There have been hundreds of people claiming to define the best form, stance and hit in order to make that perfect golf swing consistently achievable to land you closest to the hole. One such principle was made popular by Ben Hogan and it has been coined the "Ben Hogan's pane of glass" or simply put, the golf swing plane.
The way Ben Hogan demonstrated the golf swing plane in the book was through a diagram that likely can be observed online. The diagram depicts a pane of glass arising from the ball on the ground, diagonal along the golf club at an angle through which the golfer's head protrudes from, giving the image of a pane of glass resting on the shoulders of the golf player. This pane of glass is extended through to the golfer's target line and when the golf head is made to swing along this pane, you improve your chances of making good contact.
Although visualizing this swing plane may seem easy and simple to execute, it is rather quite challenging to attain. Even the best professional golf players out there have difficulty keeping their golf heads at an ideal golf swing plane. If you feel as though you've already achieved proper form and stance yet there is still something wrong with your swing, you can try documenting your swings and studying it, and determining whether you remain at a constant golf swing plane or not. See if you're able to swing your club at the same plane as it returns. Don't be discouraged if it does not. Simply make modifications to your swing. Remember you do not have to be on the exact golf swing plane, as long as you achieve a flatter or more parallel swing, you are on the right path to becoming an even better golf player.