After once learned, why do you never forget how to skate, ride a bicycle, brush your teeth, or drive a car. Repeated behaviors are "learned" by the development of pathways through which environmental signals more rapidly reach the motor nerves. These nerves are coordinated by the spinal cord, not the brain. Amazingly, many animals can run around even when much of their brains are missing. With time, your pathways become fixed. This is the reason it is so difficult to change your handwriting style, even when you try hard. Once you become proficient at Taekwondo techniques, it is difficult to demonstrate the wrong way to do them to beginning students because your brain has little control of the motor memory actions. With years of training, martial artists act and react with little conscious thought.
Do it right from the beginning
When the body learns a new skill, it reprograms the brain and the nerve pathways that lead to the muscles involved in performing the skill. Therefore, it is important that when you learn a new skill that you perform the movements slowly and perfectly, since you are programming your body to perform the movements. As you perform repetitions of a movement, the nerves, as well as the muscles, get fatigued, so the information being passed by the nerves get muddled. In fact, the amount of electrochemical voltage across a nerve decreases with every repetition. Thus, if you train while fatigued, your body will learn the new skill in the muddled form.
It takes about 300 to 500 repetitions to reprogram the body to the movements of a new skill. If the movements were learned incorrectly, it will take 3000 to 5000, ten times more, repetitions to reprogram to the correct movements.
Therefore, it is much better to—do it right from the beginning. Perform as many repetitions as you can perform properly in one training session, and never train until failure. When you begin to get fatigued, your form will deteriorate and you will train your body to perform the skill incorrectly.