1. Relaxation - The most likely culprit in this area would be your upper arm muscles. The trick is to
figure out how to relax the muscles. A good way to learn to do that is to tighten up every muscle for
a few seconds, then let go. Also, check to make sure you are not gripping the bow (bow "grip" is a
misnomer!) Tension on one side of the body can often be traced back to the other side, as well.
Don't grip the violin!
2. Right elbow height - is your elbow higher than your bow hand? They should be approximately
the same distance from the floor. A concurrent problem is often excessive pronation of the bow
hand (pushing down hard on the index finger), which leads to scratchiness.
3. Contact point, i.e. the point at which the bow contacts the string - it should be halfway between
the bridge and fingerboard, under most circumstances. The other potential problem here is the
direction you draw the bow. If it is not exactly perpendicular to the string it will "skid," which often
draws out the upper overtones, and creates a squeaky, squawky sound.
4. Proportion of bow speed to pressure - too little pressure to too much bow speed = skidding
(see #3 above). Too much pressure to too little speed = scratchiness.
5. Concept of sound - Concepts are always tricky (imagine teaching voice!) Think of yourself as
"pulling" the sound out of the string, rather than "pushing" it out. Imagine the bow is a long comb,
with many tines, that you are pulling across the string, or perhaps a piece of fine sandpaper. The
idea is to create just enough friction to make the string sing.
If you really want to fix your tone, spend much time in front of a mirror playing open strings. It
sounds boring, but if you're trying to improve, it won't be. Once you have the sound of the open
"A" try applying it to something easy - not the hardest thing you're working on. Work yourself up
gradually. Half the battle is recognizing, and disliking, the unpleasant sounds. Once you're honest
with yourself about them, as obviously you are, your ear will help you make the improvements.