My current relationship did not start out as a D/s relationship, but instead, it evolved over a few decades. Therefore, I did not have to train a submissive in the same way as someone who is trying to get a submissive up to speed in a relatively short amount of time (a few years).
So the idea of punishment is still somewhat of an abstract concept, other than I know it is a tool to train. Some articles that I have read suggest punishment should be used to help the trainee to remember her transgression through pain or isolation, while others are more nurturing (which would be my inclination). [Note that all of the articles that I have read are strongly against abuse.] All of the punishment is done with the idea of helping the submissive be submissive. And I know that what is appropriate depends on the type of submissive (brat, little, rope bunny, ...) and the type of transgression.
What I want to know is what has worked for you personally (as a Dom or a sub) and why?