The Shadow Drill
NOTE: This drill is particularly helpful stopping fat and topped shots.
First you must have a sunny day for this drill.
This purpose of this drill is to help you keep your head still during your swing. (Note: when we say still we don't mean "frozen". If your head moves and inch or two side to side that's unlikely to cause you any problems. However, we don't however want your head going up and down at all.)
Keeping your head still, in particular, eliminating any up and down movement of your head will pretty much eliminates fat and top shots. This is a great drill for the back yard too, with a plastic or whiffle ball.
Turn so you can get in your correct address position with the sun directly behind you and your shadow, specifically your head's shadow covering the ball. Let me repeat that, "Set up in your address position with your head's shadow on the ball."
(Note: If the sun isn't at an angle where you can get your head over the ball then mark the position of your head's shadow with a tee or some other object.)
Take some practice swings keeping your head in that same position (or at least close to that position) throughout the swing. After your practice swings try hitting some practice balls again watching your head's shadow on the balls.
Additional tip: When you set up make sure you keep the weight mostly on the ball of the back foot (see the Back Heel Up Drill). Also you should push forward with your back leg literally bracing into your front leg. You should feel some pressure from this pushing on the outside of your front leg. That bracing into your front leg should keep your lower body stable and relatively quiet during your swing and automatically give you a little push through impact enabling you to finish facing the target. Most golfers find this bracing into your front leg makes it easier to keep your head steady throughout the swing.