The most successful drills for this, as has been mentioned, involve fairly short active turns where the student's body faces down the hill while the legs turn beneath. Like all drills, however there are some pitfalls. Frequently it tends to promote the idea in the student that the body faces squarely down the hill in ALL turns. With that erroneous notion in their heads they try to do this in medium turns and contort themselves all up into an overly countered, mis-alligned position. (Someone referred to this already). I try to make sure sure that in the progression of developing this skill they understand that what we are really trying to promote is the notion that the legs start turning the skis and the upper body FOLLOWS the skis around during the turn, never really catching up-that is the skis are always pointing further to the right in a right turn than is the body. A drill to train this: As the student finishes a right medium radius turn she extends her right (uphill) arm and points and looks at an object on the right side of the hill more or less in line with tips of the skis. She then starts to turn her legs to the left (to start a left turn). The extended uphill arm sort of "anchors' the upper body and allows the lower to start turning independently, producing separation. Then once the turn is started the student "releases" the uphill arm and brings it down to the back of the downhill boot, which allows the upper body to "follow" the skis without really catching up.
Of course I also use the short turn drills as well, with the explanation that in short turns it IS OK to "face the body down the hill" but longer turns require the body to follow. Seems to work well.
I just back from an indoor race clinic and I also pointed out the tension in he inside arm. This will be my focus tomorrow on snow, which for me, is a trigger to functional tension. Thanks!