Years ago, in a sweaty dojo, I earned my black belt and wisdom about the art of practice from Master Dave. Master Dave wasn't a Ph.D researcher with some degree in neuropsychology but his years of experience taught him that practicing something over and over does not make it perfect.
There's a lot of information out there on how to master skills. Some ideas are misleading, such as the concept popularized by Malcolm Gladwell in his book Outliers, in which he explains 10,000 hours is the key to mastery. Other articles give you tips on how to practice without any research to back it up.
But if you want some researched advice about how to master a skill, listen to Anders Ericsson, author of Peak: Secrets from the New Science of Expertise, who has researched practice and proven what I learned from Master Dave long ago: if you want to learn something, you need to practice deliberately.
The Six Keys to Deliberate Practice
In order to improve your performance--with karate, a musical instrument, or any work related task--you need to do these six things:
1. Establish well-defined and specific goals
Have a goal of what you plan to accomplish. Otherwise, how will you know if you reached it? Mastering a fluid karate move, playing a song without mistake, or anything you want to improve, as long as it is specific and something beyond your current ability.
2. Break your task down into parts and make a practice plan
While a karate punch may look like one seamless move, it has many moving parts. Break your own task down into its different areas. Then make a specific plan of when and how long you will practice on each part.
3. Give each part your full attention
Passive practice does not lead to mastery. You won't become like Master Dave by multitasking. You need to be focused on each movement to get the most out of each specific task. Practice slowly at first, so you can master each section; then put it all together into one seamless action.
4. Get feedback from a master
No one masters a skill by themselves. I would have never become a black belt without Master Dave correcting my punches and kicks. An expert outsider can see your errors and help correct them. Practicing the wrong way will only make you master it the wrong way. To achieve perfect performance, you must first practice perfectly.
5. Stray out of your comfort zone
No one ever became a master by doing what they already knew. Go one step away from your ability--and no farther--to stretch your expertise. In karate, we start with simple moves and then work up to combinations. Learn a slightly more complicated part or move. But be careful, if you go too far and fail, you might be discouraged (and maybe hurt yourself).
6. Maintain your motivation
Motivation is mastery's fuel. To maintain your motivation to change, according to Chip and Dan Heath of Switch, you'll need to have three things: the emotional, logical, and logistical reasons to continue.
Logistically, maintaining motivation is simple as finding the most convenient time and place to practice. If you have too many logistical obstacles--getting up too early, driving out of your way, an uncomfortable practice area--you'll give up. The fewer barriers you place to doing your practice, the more likely you will do it.
As for emotion and logic, you'll have to do the hard work and figure out the reasons why you want to become a master of something. Your emotional reason may be a negative incident. For example, Michael Jordan was cut from his varsity team, which motivated him to work hard to become better. Or maybe it's something positive, such as a teacher's or boss's glowing praise about your skill that drives you to work hard.
Whatever it is, you'll need to have these answers or you'll never devote the time to practice perfectly and master your skills.