One thing that my Son has focused on when working on his swing has been "the finnish". Basicly when he completes his swing he freezes and has a series of "check points" to inspect, such as 1) Balance, 2) front foot 3) not overrotating the backfoot 4)Powerline 5) clubhead tilt and 6) head. The logic behind this is that if the body ends in the correct position, then it had to go through some correct motions at full speed to get there. At the end of the swing it is easy to see where you end up by freezing and checking the batters position at followthrough.
Another drill that works very well for us has been "Dynamic Pepper". I described this once before but will try again.
The basic goal is to master Club Head Control, so it really works the forearms and wrists. It is a Soft Toss Drill.
First, the batter is tossed the ball and must hit the ball squarely on the sweet spot and freeze his bat. The ball should jump off sharply toward the direction the pitch should optimumly be hit (outside of the plate the ball should go between 2nd baseman and 1st base) Secondly with the same pitch location the bat strikes the ball then retracts the bat immeadiately following the identical swing plane. Thirdly the hitter using only upper body with feet planted swings through and drives the ball and lastly the hitter takes a full swing.
The location of the soft toss pitch is the same through each of these "swings", perhaps 5-10 with each version, eg. freeze , retract, upperhalf, full swing. The back hip should explode and point in the direction the pitch should be hit as well.
So...soft toss to outside of plate and the hitter should master driving the ball opposite field. Then work middle of the plate up the middle, then inside for pull.
Soft toss works well for this because you should be able to place the ball consistantly where you want it allowing the batter to focus on one thing at a time, such as opposite field.
The Dynamic pepper drill works very well.
Here is a video of my guy doing a the "Dynamic Pepper" drill. Not perfect, and that is the point, with the drill you can easily tell when he mis-hits and gets a bit under the pitch, making it easier to adjust and move forward. Only the "freeze" and "retract" positions are shown.
http://www.getmynameoutvideo.com/DoyleBaseball/Pepper.wmvHere is another player doing the soft toss drill and freezing at the finnish.
http://www.getmynameoutvideo.com/DoyleBaseball/SoftToss.wmv