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, four weeks later, flossing automaticity. Automaticity was also measured at eight-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 eight-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.