Gotham Golf Blog has an excellent drill posted on it. It’s called the four finger swing.
I practiced with a variation of it today(the 6 finger swing) and I found it extremely helpful. You hold the club with the thumb, index, and middle finger of each hand in a golf grip. That gives you just enough leverage that it’s easier to do a full swing, but it gives the club enough freedom to move around.
What I found doing this was that when I did the 6-finger swing, the club would set in the right place at the top of the swing on plane and in the right place. Because you can’t manipulate it as much with your hands, the club follows a natural path. It gets on plane quickly and stays that way.
On the downswing, again because you only have 6 fingers on the grip, you can’t manipulate the path. The club takes the natural path down which, for me kept it beautifully on plane.
After doing a few 6-finger swings, I then took my regular grip trying to feel the swing like the the 6-finger swing felt. It felt very natural and I was able to fully accelerate the club head. When I looked at the results on video, I was suprised at how well, the 6-finger swing, transitioned to my regular swing.
This could be a great drill to do whenever you are on the course and the swing seems to go off a bit. A few 6-finger swings could put you back on track.