The fourth group, which had trained most heavily in mental preparation and visualization exercises, showed better improvement than the other three groups. By practicing of course, but not in the traditional sense. First, sit in a quiet area. Now, choose a section that has caused you difficulty. Don't be discouraged if this takes a while.
Visualization takes a lot of concentration at first, but gets easier with time.
Now move on to hearing yourself play with great expression and with the most beautiful tone. When you pick up your instrument, continue the same thought process. Keep your attention on the image in your mind. The time for assessing how well you've done comes after you've played, not during. You cannot simultaneously make a statement and ask a question.