Lazy and insidious by their very nature, peasants must constantly be disciplined. And not just after their crime, but before it! That way they shall remember the punishment for misbehaviour.
Of course, we must still keep them in (relatively) good shape, or else they shall lose the strength to work. Therefore, I advise moderation. A few lashes a day or one proper beating per week (preferably before mass, so that the preventive lashing is followed by a mea culpa and penance right away).