This study investigated the change in preservice teachers' (PSTs) mathematics teaching self-efficacy through a mathematics methods course and revealed the best practices that helped the PSTs develop a higher self-efficacy and a growth mindset for teaching mathematics. We collected data through a scaled-response questionnaire to measure PSTs' mathematics teaching self-efficacy, and an open-response survey to explain what, in the methods course, facilitated the development of PSTs' (N = 92) self-efficacy and mindset for teaching mathematics. We conducted paired samples "t"-tests and descriptive analysis to examine the scaled-response questionnaire data and used an open coding process to analyse the open-response survey data. We found that the PSTs significantly improved their mathematics teaching self-efficacy throughout the methods course; and identified five practices that contributed most to the development of self-efficacy and growth mindset in teaching mathematics, which included inductive teaching, differentiation, supporting productive struggle, problem solving, and probing thinking questions. This study sheds light on how to strengthen teacher education programs to better prepare PSTs in teaching mathematics to elementary and middle school students.