This may sound funny, but just PRACTICE. You should be practicing your free throws at least 3-5 times per week. And when you are practicing, you should be going through the same “routine” every single time. This is something all college and NBA players have for their free throw shot. Whether it’s 2 dribbles, spinning the ball in your hand, or taking a deep breath, make sure you go through the same exact routine before every free throw shot. This will develop muscle memory and increase your free throw percentage.