Adventures with 'other' instruments...
Here's what I do to get my classes to relax. We start out with just sitting there with your arms and hands complely relaxed with your wrists resting on your legs. Then, LIGHTLY grip the bow, jsut enough so you can lift it up. Put the bow on the string and just bow the open string, concentrating on keeping your shoulders relaxed, your posture good, breathing deeply . Draw the bow over the string and see if you can get a steady tone. On left hand just bow the open string and concentrate on staying relaxed, gripping the bow very lightly and moving the bow at a steady pace.
Next, keep your elbows realxed and close to your body. Now see how far you can move the bow back and forth and not move your arm at all, kind of like Surrty County fiddlers. This is all wrist movement. If you have to, have someone hold your arm so it can't move and see how much movement you can do with just the wrist. Watch that your shoulders don't stiffen up. Try this and see if it helps. If not, let me know and we can try some other things.