With the widespread use of new information technology, calls have been made for leadership training research to examine if the effects of leadership training can be boosted over time by sending text messages or e-mails, in order to reinforce the lessons learned in training interventions. Based on media synchronicity theory (MST), the purpose of the current study was to contrast the usefulness of two booster activities: traditional telephone coaching, and exercises sent by e-mail. Managers were randomly assigned to one of the two booster conditions. We then used a mixed methods design including both quantitative and qualitative data to evaluate the boosters. We obtained the quantitative data from 20 managers and their 323 employees at four time points: before, during, and after the training. We also used focus group interviews to evaluate managers’ experience of both booster activities. The quantitative analyses indicated statistically significant differences between the two groups after the second session, where managers in the e-mail group scored higher on readiness for change and perceived applicability of the training. The latent growth curve analyses indicated a statistically significant increase in employees’ perception of managers’ autonomy support. The qualitative data suggest that the e-mail booster was considered informative and flexible but also time consuming, while the telephone coaching was perceived as flexible but somewhat unstructured. Our findings suggest that an e-mail booster could be a cost-effective alternative to reinforce lessons learned in leadership training.