Depending on what exactly you want to train you have to alter your methods your rewards and your punishments.
Especially hard to train are for Example Masochists. You somehow have to connect the pain, which the brain regards as extremly negative, with enough positiv emotions to make the brain regard it as positive.
The problem is that whenever you feel pain the brain dislikes it.
As far as I know there is no reliable way to train a masochist. Normally there are several mistakes which lead to a person skilled at acting who hates you because you made this person suffer through much or the person loses the will to live because everything positiv is connected to pain or a person who learns to absolutly ignore pain and Have fun despite it. That is one of the rarest cases. The best method I have come across is to make the slave feel just a little pain. Soft spanking while giving a sexual reward, Teasing after they got sore, punishments without involving more pain like taking luxuries away from them or ignoring them and making them feel little amounts of pain constanly.
After some time you spank them during training testing whether the slave gets aroused from it. If the slave aroused from the pain you can slowly get the level of pain up in a constant rate.
After some time you can use stuff others see as punishment as reward like riding the horse with a vibrator egg inside for 10 - 20 minutes or even longer depending on the slave . At that step you should add a line to their mantra. If possible let her listen to the experiences of other masochists at a similar or better a slighly higher stadium so the slave doesnt thinks of it as strange or unacceptable.
It is the best to be not to open about trying to make the slave a masochist in the beginning since most people see it as unnatural. Only start asking whether they like a little pain during sex. That is something many can admit after some pain training. After accepting they like little amouts of pain during their sexual fun it gets easier to make the slave accept pain. Always remember it is easier to train a slave that accepts the trainings, is dependent on you and trusts you.
Some even say it would be better to wait with the pain training till after the normal slave training. I have to admit that the slaves who have already given themselves to you are much easier to control but they already have a stable slave mindset which you could disrupt through changing their routine or breaking their trust through pains they accept because of their mindset despite hating it. That's why I dislike this method.
Also regarding slaves in higher Positions working for you. It can work. It can give you certain benefits. But you can never be sure that a slave you trained won't betray you. Even if you make your slave regard you as a good which the slave owes his life and soul to and who should never be betrayed or taken advantage of there could be a other person who doesn't get scared by your slaves behavior and tries to charm her. There will be a chance the person devotes himself or herself fully to your slave and that way gains your slaves trust. That would break up the basis of the training: Dependence and Trust.