One thing that Ted Fort pointed out to me that really helped my underplane issues was Extensor Action. My right elbow was getting a little too "close to me" coming down which brought my hands and clubshaft underplane.
Really get a "wide" feeling via Extensor Action as you are coming down. It'll feel like the club is way out in front of you.
Thanks, I know this is an issue for me...when I'm not hitting the ball well, my right forearm comes in too high and I don't have that "tight" feeling. After watching the TT videos I'm much better with this.