The mirror might be the best training tool.
I use a cheap framed closet mirror. I set a dowel in iside the frame of the mirror at the exact angle as my right foreamr at address.
I practice the right forearm takeaway over and over. At the top my
hand(s) match the dowel plane line.
Another great plane drill I got from watching Chuck's video clip.
http://chuckevansgolf.com/media/righ...f_approach.mov
Watch Chuck's right forwarm angle clip. Set the left hand dowel as Chuck does to match the impact right forearm angle. Swing a dowel in your right arm to the top. The left and right arm dowels should line up perfectly on plane. Simply the best way to understand and ensure the correct plane line.