Poor posture leads to your body parts not being in the correct position to rotate around your spine. This leads to over-rotation, where your body rotates all at once instead of as correctly needed in the swing.
Teach kids that with irons you must hit down on the ball to make it go up because the ball gains backspin by rolling up the face
Ben Hogan dedicated nineteen pages in his book to the grip which is mainly in the fingers and palms