Practice hard parts longer.
You have to practice longer on things that you can’t play. You know, you’re around the house or at school and you want to show off. You want everybody to be proud of you, so naturally you’re going to play whatever you can play. Generally, that will be the easier things. So when you get to hard part, that’s when you really have to buckle down, concentrate, and dedicate yourself. Don’t be afraid of confronting your own inadequacy. You’re a person, not a machine. You can’t be perfect. For some people the intricate, fast parts aren’t easy, and the slow ones are difficult. Everyone is different.So things are so hard that you might have to practice them hour after hour after hour. You know, it’s like in basketball. You see players go out on the court and do all kinds of fancy dunks and things that they’ve worked on, but then they go to the free-throw line, and they can’t make the free throw. It’s because they didn’t practice what they couldn’t do.Your practice schedule should reflect your particular strengths and weaknesses. That means practice the things you can’t do longer than the things you can. This is always a difficult discipline, because we want to sound good. We have to train ourselves to endure something bad for that “short” time it takes to improve. When you have a problem, confront it. That’s the one way to solve it, not running away and avoiding it. Playing only what our friends and even we ourselves will praise leads to one-dimensional music, as it can also lead to a one-dimensional personality. Our practice schedules should reflect a confrontation with our deficiencies, with the constant goal of improvement.
Wynton Marsalis, from “Marsalis on Music” p. 128-130