A couple of years ago I did a post about learning proper knee movement with a drill using a basketball. You can find that post here.
But I recently found a drill over at golftipsmag.com that helps with the "rolling" foot motion that can help you get that knee movement correct. It's one of their Quick Tips called Fancy Footwork by PGA teacher Mike Corcoran, and it's a simple way to help you keep a more consistent height during your downswing.
That helps you make better contact with the ball. Hitting the ball fat or thin less often is always a good thing!
Essentially, you roll your ankles -- both of them -- during your downswing. This helps you keep the same amount of flex in your knees and also helps you keep your trailing heel on the ground a bit longer. That not only improves things like balance and weight shift, but it also helps you keep your swing on line for a longer time. That gives you a chance to hit the ball straighter as well.
Give this drill a try. At the very least, you should find it helps improve your short game.