Don't lapse into auto-pilot with it. Keep bow on the sound point, not letting it slide up and down. In order to have an even sound, always approach the strings with your left hand fingers at the same angle. Don't change your angle of approach as you move around the fingerboard. Beethoven C Major Sonata - Starker plays the entire first phrase on the D string.
Extensions are an unnatural act.
Do them only when necessary. If a note is too loud, strengthen the note before or after. A tense thumb is an international problem, in both hands. Release the thumb during trills. To learn to breathe naturally, study singers.