While many factors influence the plane on which you swing (e.g., spine angle, right forearm position at the top of the backswing, degree of arm/body connection, and amount of knee drive), there is a great training aid that can help improve the swing plane without excessive focus on these mechanical aspects.
The Dual Track is a plane and path trainer, which can be adjusted to any individual as to the flat or upright preference of their swing. Once set, the player simply swings back along the two guide rails. As the club shaft passes the waist high point of the backswing, the entire Dual Track collapses behind him or her, allowing a full, inside downswing. This unique characteristic actually allows the player to hit balls without the risk of striking the device during the swing.
Continued practice with the Dual Track, especially under the guidance of a teaching professional, will help to develop a correct, consistent plane. The many benefits of improved contact, direction and distance will be well worth the effort.