We provide rare evidence for how a temporary incentive can kick-start an improved habit. We conducted a natural field experiment involving 70,000 households in an urban area. We study the extent to which they separate their waste, a behavior that is prone to habit formation. The incentive consisted of a letter informing households that disposal of non-separated waste is illegal and punishable by a fine, followed by an intensive and highly salient crackdown of four weeks. The crackdown had a large and instantaneous effect on household behavior. Most of the effect was still apparent up to seven months later. The stable, higher rate of separating waste is likely to have been sustained by a new, improved habit: crowding out of intrinsic motivation by the transactional motive to avoid punishment is found to be only transitory. In contrast to the threat of punishment, two behavioral interventions, one conveying the injunctive norm, another one the descriptive norm, are not found to have any effect.