Summer research opportunities for undergraduates, such as those supported by the National Science Foundation's Research Experience for Undergraduates (REU) program, can be critical experiences that help persuade students to pursue research through graduate studies. Studies analyzing the key actions of successful mentors are scarce. The goal of this study was to explore how hypothesized ''key actions'' of mentors correlated with student perceptions of mentoring and of overall program quality, students' scholarly output resulting from the REU, and the influence of the REU on students' decisions to pursue graduate school. Students who participated in 11 REU programs at a large Midwestern US university in summer 2012 were surveyed in spring 2013 about their experiences in the program and with their primary research mentor. Results suggest that the key factors hypothesized to be associated with good mentoring correlated with students' ratings of their relationship with their mentor, students' overall program ratings, students' scholarly output resulting from the REU, and the influence of the REU on students' decisions to pursue graduate school. The six ''key actions'' hypothesized to be associated with good mentoring were significantly correlated with student experience in an REU program. Although none of the six actions is particularly complex, taking these actions is crucial and non-trivial. Prospective REU mentors could be informed about these key actions during mentor training activities. Future research could examine these six dimensions across postsecondary institutions and in mentoring relationships outside of REU programs.