Expertise in any domain is attained through extensive practice. The best practice methods depend on several factors, among which are the three primary phases of training: acquisition, retention, and transfer. In this chapter, we consider nine principles that promote skill learning, all of which have considerable empirical support. Four of the principles involve complicating learning, whereas five involve simplifying it. Complications that have been shown to be beneficial include desirable difficulties, cognitive antidote, variability of practice, and strategic use of knowledge. Beneficial simplifications include focus of attention, mental practice, rules, functional task, and stimulus-response compatibility. Which principle or set of principles to use in a particular training situation necessarily depends on which particular task is being learned and which parameters of the task can be manipulated. Nevertheless, the principles provide useful guidelines for consideration when designing a training regimen to optimize skill learning.