Masters must have clear set of goals in mind before training their slave. They must also have some prior knowledge of slave traing before they attempt to take in a submissive.
Just like owning a pet, owning a slave is a HUGE responcibility and it requires a lot of work. Having a slave requires time and patience on the part of the Master. He needs to understand that a M/s relationship is a 24/7 thing. If he can not fully commit to it, he should not attempt to train someone as his slave.
Just like with any other job, a successful Master is one who truely loves what he is doing. If he does not enjoy the relationship or is only involving himself in it to please someone else, it will not work.
A Master must exert control over himself. If he can not control his own habits, he can not hope to control those of his slave.