Forming habits – broadly defined as learned automatic responses to contextual cues – is proposed as a means to health behaviour maintenance. Habits form through context dependent repetition, but additional variables may affect this process. This work aimed to investigate additional predictors of habit formation. The potential predictors investigated were: placement in a routine; attraction versus health information; perceived reward and its impact; and individual differences. The study investigated habit formation for dental flossing and taking a vitamin C tablet daily, in a sample of 118 participants from the general public. Eighty participants received an online intervention for vitamin C tablets at the start of the study. All participants received an intervention for flossing after four weeks, randomised to be based on attraction versus health, and to advise flossing before or after brushing. Self-reported behaviour, habit, and motivational variables were measured every four weeks, for sixteen weeks. Habit strength and behaviour increased for both target behaviours. Neither behaviour nor automaticity were affected by the attraction and health interventions, or placement of flossing before or after toothbrushing. Experiencing the behaviour to be rewarding led to greater automaticity, but was not mediated by behaviour frequency. Reward strengthened the behaviour-automaticity relationship, as predicted by theories of reinforcement. Pleasure and intrinsic motivation also acted in this way, but positive attitudes about the behavioural outcomes did not predict behaviour or automaticity. Rational, rather than experiential, thinking style was associated with weaker habits, and higher prospective memory ability predicted greater initial gains in behaviour, followed by stronger habits. The results suggest that there are experiential and trait factors which affect the habit formation process and which can reinforce habits, beyond their impact upon repetition. These findings advance habit theory and lead to suggestions for intervention development, such as recommending encouraging intrinsic motivation, and tailoring interventions according to individual dispositions.