Swinging the golf club on plane is a term most golfers have heard, but I've noticed its something that most golfers don't really understand what swinging the golf club on plane really means. This Hank Haney video describes what the swing plan really is. Its important to note that the plane is player specific.
He talks about different teachers and their thoughts on the correct swing plane. Players like Ben Hogan advocate a swing plane from your shoulders to the ball. It seems that this school of thought has been changed slightly over the years. Haney and many teachers now advocate a swing plane of the shaft line. Without a doubt both can be successful, but only if the plane is maintained throughout the swing.
I believe working on your swing plane is an important part of practice, and swing maintenance. Watching your swing in a mirror is an easy way to see if your staying on plane. Or you could work with a partner, or even better take and review a video during practice. Anytime, I am looking at my swing plane, I always start with looking at my setup and posture. If the setup and posture are not correct, there are likely swing plane compensations taking place. So, it is vital to make sure these two are in line before looking at your swing plane.
Lastly, something I find helps me when practicing my swing plane is visualization. I will picture an invisible circle around my body. Or a sheet of glass.
Check of the video and practice your swing plane from time to time as part of your swing maintenance.