Studying in-situ technology use over time can be difficult and this is especially so when considering technologies such as activity tracking devices explicitly designed to be unobtrusive. Yet understanding activity tracking in practice is crucial, as tracking technologies become important tools for health promotion and health insurance programs. In this paper, we describe a method for a longitudinal participant-driven photo elicitation study of activity tracking. During the five-month long study, our drop-out rates were low and we observed idiosyncratic practices with lapses and particular use patterns among participants along with significant self-reflection on activity tracking as a practice. We describe our method in detail, discussing the necessary adaptations for the study of activity tracking practices. We offer our experiences of benefits and challenges of this process, and suggest points for consideration for future studies in the area.