I find that jerking your stroke is usually a result of a shot that's outside your skill level or if you're really nervous.
The best trick is to find a shot that you struggle with, like a long straight draw, for instance and do a progressive practise. So you start at a comfortable distance and just focus on keeping the stroke perfect, then gradually increase the distance. Do it with any shot you struggle with. The point is to remove the tightness from those shots, gradually increasing speed while not allowing any jerking of the stroke to occur. ...