We, like other educators, are keen to develop the next generation of visualisation designers. The use of sketching and low-fidelity designs are becoming popular methods to help developers and students consider many alternative ideas and plan what they should build. But especially within an education setting, there are often many issues that challenge students as they create low-fidelity prototypes. Students can be unwilling to contemplate alternatives, reluctant to use pens and paper, or sketch on paper, and inclined to code the first idea in their mind. In this paper we discuss these issues, and investigate strategies to help increase the breadth of low-fidelity designs, especially for developing data-visualisation tools. We draw together experiences and advice of how we have used the Five Design-Sheets method over eight years, for different assessment styles and across two institutions. This paper would be useful for anyone who wishes to use sketching in their teaching, or to improve their own experiences.