Respondent-driven sampling (RDS) is widely adopted as a method to assess HIV and other sexually transmitted infection prevalence and risk factors among hard-to-reach populations. Failures to properly implement RDS in several settings could potentially have been avoided, had formative research been conducted. However, to date there is no published literature addressing the use of formative research in preparing for RDS studies. This paper uses examples from Banja Luka, Bosnia and Herzegovina; Bangkok, Thailand; Podgorica, Montenegro; and St Vincent's and Grenadine Islands, Eastern Caribbean; among populations of men who have sex with men, female sex workers, and injecting drug users to describe how formative research was used to plan, implement, and predict outcomes of RDS surveys and to provide a template of RDS-specific questions for conducting formative research in preparation for RDS surveys. We outline case studies to illustrate how formative research may help researchers to determine whether RDS methodology is appropriate for a particular population and sociocultural context, and to decide on implementation details that lead to successful study outcomes.