To improve the overall success rate, you need to ask people to not only make attempts at home, but to also practice the techniques and entire procedures for phase entry. This is especially true of longer courses in which the student has lots of time for additional activities. The better the techniques work during practice, the easier it is to get results with them during an actual attempt, the more you will understand them at a fundamental level, and the better acquainted you will become with them.
Various literature on the topic can also be assigned as reading homework. Practitioner accounts of phase experiences are especially well suited for this. They can be found in large numbers on our websites and forums. Reading them will substantially increase motivation and provide a large number of real-life examples of how out-of-body experience is mastered.