Habits involve regular, cue-triggered routines. In a field experiment, we tested whether incentivizing exercise routines-paying participants each time they visit the gym within a planned, daily two-hour window-leads to more persistent exercise than offering flexible incentives-paying participants each day they visit the gym, regardless of timing. Routine incentives generated fewer gym visits than flexible incentives, both during our intervention and after incentives were removed. Even among sub-groups that were experimentally induced to exercise at similar rates during our intervention, recipients of routine incentives exhibited a larger decrease in exercise after the intervention than recipients of flexible incentives.