Objectives Habit formation has been proposed as a means to promote maintenance of healthy behaviours, but there have been few investigations into how habits are formed. This exploratory study sought to model determinants of the formation of a dental flossing habit, including placement of the behaviour within the routine (before vs. after tooth‐brushing), past behaviour, prospective memory ability, and motivational factors. Design and Method All participants (N = 50) received a motivational intervention designed to initiate behaviour change and habit formation. Half of the participants were instructed to floss before brushing, and half after. Participants subsequently self‐reported flossing behaviour daily and, 4 weeks later, flossing automaticity. Automaticity and flossing frequency were also measured at 8‐month follow‐up. Results Participants with stronger prospective memory ability, higher levels of past behaviour, and a more positive attitude flossed more frequently during the study. Stronger automaticity was predicted by positive attitudes, and increased behaviour frequency during and prior to the study. Those who flossed after brushing (rather than before) tended to form stronger flossing habits and, at 8‐month follow‐up, had stronger habits and flossed more frequently. Conclusions Habit forming interventions might usefully consider features of everyday routines and how behaviour may be reinforced. Suggestions for further research using more methodologically rigorous designs are offered. Statement of contribution What is already known on this subject? The formation of habit – that is, a learnt automatic response to contextual cues – requires initiation of a behaviour and repetition in a constant context. A recent formation study showed variation in habit strength despite equal repetitions, indicating that factors other than repetition may be important in habit development. From studies of routine behaviour, the boundaries between sub‐routines are characterized by different processes than the middle of sub‐routines, suggesting that placement of behaviour within existing routines may affect the likelihood of habit formation. What does this study add? Greater prospective memory ability predicted more frequent dental flossing. Initiating flossing after rather than before tooth‐brushing promoted stronger habits. Positive attitudes impacted directly on habit formation, independent of behaviour repetition.