If there's one tip i give to both slicers and weak hitters, it's to improve their left-hand grips. What I see is the grip sitting in the palm of the hand, not in the fingers. This inhibits the player's ability to hinge the left wrist (for power) and to square the clubface at impact (for control). A sure sign that you have this fault is a wear mark on your glove at the heel of your hand.
To improve your grip, adjust your left hand until you can see two knuckles at address. If you see only one knuckle, your grip is too weak. If you see three or four, you've overcooked it. Using a marker, put dots on your first two knuckles (right) to ensure you achieve this position consistently. (Yes, you can play with the marked-up glove in competition.)
This improved left-hand grip will encourage the free hinging and unhinging of the left wrist to maximize speed. It will also help rotate the clubface on the downswing so it's square at impact.
David Leadbetter* is a Golf Digest Teaching Professional based at ChampionsGate near Orlando. He operates 26 golf academies worldwide.*