Gentle Public Humiliation
It is a well known principal of psychology that people behave differently if they think others are watching or will find out what they are up to.
Publication of broken results with an indication of who or what most likely caused those results will help honest people stay honest.
Feedback should be as fast as possible (see Regularly Scheduled Builds) for maximum effect.
As much as possible the humiliation should be from a neutral party (a computer) and be presented in a gentle or humorous manner so as not to inflame hostilities in people who are probably already stressed out.