In mobile Delay Tolerant Networks (DTNs), messages are routed in a mobility-assisted manner. Therefore, the willingness of nodes to relay messages for other nodes plays a significant role in the routing process. Moreover, since the resources in mobile devices are generally limited, carriers of mobile devices may be unwilling to relay messages for other nodes in order to conserve their scarce resources. Furthermore, recent studies on the impact of selfish behavior in DTNs show that the performance metrics (i.e., delivery ratio, delivery overhead, and delivery latency) are severely influenced if a major portion of the nodes is selfish. A number of strategies have been proposed to prevent the impact of selfish behavior. It is therefore necessary to give an overview of the representative strategies. In this paper, we first classify the different types of selfish behavior. The existing strategies for preventing selfish behavior are traditionally classified into three categories: barter-based, credit-based and reputation-based. We then survey some representative strategies in the literature belonging to each category. Finally, we conduct experiments to compare the performance of these strategies for preventing different types of selfish behavior.