Practice does NOT make perfect - Scientists believe that striving to consistently repeat a physical action (like shooting a basketball free-throw or serving a tennis ball) is unnatural. Our bodies and brains are simply not designed to repeat the same motion in the same way over and over again. Variation invariably sneaks in, and the culprit for the inconsistency is the brain. It simply won't let you perform the same action in an identical way over and over again.
So the lesson is not to focus on the same physical action but the same outcome. Slight variations in form are a good thing and more natural to the body. Keep you eyes on the prize, not on perfect, repeatable form.