Another inexpensive, but quality training aid is the Pure Ball Striker (that little orange thing on the grip in the pic above) created by Jeff Evans. The PBS can attach to any club and grip and teaches the golfer how to correctly use and feel Homer Kelley's Pressure Point #3, the right index finger (for righties).
Here, Mr. Hogan explains it better than I do.
The lack of understanding of PP#3 is common for many golfers since there's a tendency to cast or flip the club and after impact when the club should be more in line with the left arm, golfers tend to not properly use PP#3 and get the club more in line with the right arm after impact.
I've found that this device does help just about everybody to some degree. It helped me a little bit and I still use it when I'm getting too handsy and over-accelerating the clubhead and not using my pivot to hit the ball.
Another great aspect of the PBS?
It's only $13.
You can find one at their Web site at http://www.pureballstriker.com/.