Repetition!
After you are done learning a piece of music, or even a small section from a piece of music - a series of notes, or one measure or one line - repeat it.
Then, repeat it agin. And again. And again.
You get the picture, right?
Don't move on to the next section, measure or line until you have learned how to play that one perfectly. Once you can play it perfectly, repeat it! Play it perfectly many more times to make sure you will never ever have to "learn" it again.
Challenge yourself during a practice session to play something (whatever you are working on) perfectly 10 times before you stop for the day.
