In this research we describe the use of human-centered design principles to engineer persuasive appointment reminder messages to encourage patients to attend their appointments. Patients' failure to attend clinic appointments often leads to poorer health outcomes and increased healthcare costs. Based on a model of persuasive messaging, we develop appointment reminder messages using a human-centered design approach. We demonstrate the value of a human-centered design methodology in the iterative design and refinement of prototype messages through the use of paper mockups, surveys, interviews of different users, and focus groups. In future work, the persuasive appointment reminders developed using the approach will be evaluated in a randomized controlled trial to test their effectiveness in reducing appointment non- attendance.