Make for certain the shoulders are cocked before throwing a left hook - in turn, the best time to throw a left hook is after a straight right is thrown - also, I've noticed a 50/50 split on how to position the hand upon throwing a hook, palm facing down OR palm facing inwards (towards you) - I like to teach with the palm facing in because it seems it's less stress on your shoulder, it offers more of a whipping motion and doing so provides a longer reach - twist your hips when throwing a hook, dip your front knee inwards toward your opposite heel & slightly fall back onto the back leg as your upper body turns into the hook.