Check Your Left Wrist
Golf instructors have argued endlessly about the different ways to achieve that goal, but I believe one way to make it easier is to swing with the clubface square to the arc of the swing.
What does that mean in practical terms? I'll use a checkpoint as an example. When you look down at the clubface at address, "square" is relatively easy to see. It's harder to see it when you're at the top of the backswing. But if you get the clubface open or shut at the top—by cupping or bowing the left wrist (or having a grip that's too weak or too strong)—you're going to hit a lot of high slices or low hooks.
Check your clubface at the top. If it's vertical (or perpendicular) to the ground (top inset), you're cupping your left wrist and opening the face. That can result in a slice. If the face points at the sky (bottom inset), you're bowing your wrist and closing the face. That can cause a hook. When the face is square, the left wrist is flat, and the face is parallel to it (above). From there, it's easier to swing to square at impact.
Hank Haney is a Golf Digest Teaching Professional and runs the Hank Haney International Junior Golf Academy in Hilton Head Island.