You gotta balance rewards and punishments but above all educate. You're teaching her how to become a sex slave. You need to teach her what is expected of her as a slave in a constructive manner. Don't punish her for failing to succeed at a task, only for lack of effort, and don't reward her for simply trying, she must accomplish something.
When she does well in her training reward her so that she subconsciously associates good slave behavior with happiness. Slowly lessen the intensity and frequency of rewards when she earns she so as not to spoil the slave, you don't want to have to reward her for every little thing once she is trained. A well trained slave expects no rewards.
When she disobeys or intentionally does poorly she should be punished so she subconsciously associates disobedience and insubordination with unpleasantness. Slowly increase the intensity and frequency of punishments when she disobeys, this keeps her from regressing once she is used to life as a slave. A well trained slave knows she deserves her punishment for misbehaving.
Balance is important, you don't want a spoiled slave who expects rewards simply for doing what is expected, and you don't want a scared broken slave who is afraid of her own shadow. The absolute worst thing you could do is reward and punish your slave inconsistently, lest she believe you to be a capricious master and her actions don't matter. She needs to know instinctively that as long as she is a good slave she will be all right and need not fear anything.