Swinging in front of a mirror is the best way to see immediate feedback. Swing facing the mirror and simply watch your head. Swing with force and bat-speed and see how much your head moves. Now try to swing with as much force and bat-speed with less and less head movement. Create a habit and a feel for this every day.
This is much like the shadow drill that was talked about but it can be done at any time of day.
Keeping the head quiet is huge when you are trying to "slow the game down" and see the ball better.