One of my favorite exercises to to help focus and calm my mind before I practice is to play a simple scale paying attention to the division of bow. I play each note of the scale for four beats and try to play each of the beats for exactly 1/4 of the bow and play at the same dynamic throughout the entire length of the bow. If this comes easy (every day of practice is different and some days my mind can barely focus enough for this “simple” task), then I pause between each note, place the next finger down and then play it. Separating the bowing from the left hand fingering helps ensure my fingers aren’t following my bow and is allows for quality assurance of my mental state before the meat of practice begins. If I can’t slow down enough to separate the left and hand, it usually means I’ll have a hard time being analytical in my approach to fixing problems.