Driving
Keep your left wrist flat
The most important part of the golf swing is the moment of impact, obviously. And though there are a lot of different swings that can produce solid impact, it's undeniable that there's only one kind of impact that can produce a particular kind of ball flight.
The important thing to remember is that with a neutral grip, the back of your left hand represents the clubface. If the back of your hand is facing the target at impact, that's where the ball will go.
Good players not only have the back of the left hand aimed at the target, they also keep the wrist flat, like it is in the picture above. That way, you're trapping the ball between the clubface and the ground and preserving the true loft of the club -- one of the keys to a powerful hit.
If the wrist cups, you are adding some loft through impact and giving up that ball-turf compression.
Practice hitting balls focusing just on your left hand -- not on your backswing, weight shift or anything else -- and you'll start to feel the connection between your hand's position and the trajectory and direction of your shots.