The answer can be summed up with one word: Prepare.
This particular student (like so many others - including myself) has a habit of correcting things after the fact. If a note is out of tune, he hears it and adjusts his finger. If the tone is less than ideal, sometimes he'll go back and try it again. Sometimes not. But that alone does not guarantee that the problem will happen again in the future.
What that student needs to do when he practices is to apply a strategy that can be helpful and productive in just about every circumstance, and which I've heard teachers prescribe in a number of different ways:
- Stop. Prepare.
- Place then play.
- Fingers. Bow. Go.
- Feel the note before you play the note.
1. Work in chunks. Pick a unit of music that addresses one issue at a time (probably no more than a few notes).
2. Identify what needs to be done to play a given 'chunk' as you would like it to happen in performance.
- Where should you be in the bow?
- Where should the bow be on the string?
- How much bow should you use?
- How should you balance your body/set up your posture?
- How do you adjust the balance of your left arm to go from one note to the next?
- How do you handle a string crossing?
- How can you eliminate excessive tension?
- How can you eliminate excessive motion?
4. Only then are you ready to play it.
Notice, there is much more thinking in the process than actual playing.