To put my own two cents in about the side conversation about volleying off a wall...I used to do this all the time when a wall was one of my usual practice partners. What I would do was drop hit a ball from a distance that would get me a ball around knee level, step in and volley it just over the line marking net height, and then move forward to volley the next ball out of the air. I would do this repeatedly while constantly closing towards the wall, keeping the ball going until I was practically on top of it. I'm not sure if that really helps with clean contact, but it definately works your volley footwork, helps you keep the ball low over the net, and when you get close definately challenges your reflexes.
As for clean contact...I have found great results both in my own game and the games of my students using a finish out in front of the body as a learning/teaching tool. I've totally ripped it off from Robert Landsdorp, so if you want more info on it look around online for some of the things he has written. Basically you hit a normal forehand, but follow through straight out in front of your body, both arms extended, the hands finishing around eye-level with the racket vertical...no wrapping over your shoulder. I usually do it first from a dropped ball, then a short toss, then a lightly fed ball, and then a full rally. After you do this a little while you then go back to your normal finish. It really emphasizes extension through the hitting zone and clean, square contact with the ball.