Anonymous communication systems built on P2P infrastructures using anonymity forwarders are frequently affected by the churn problem, i.e. frequent joins and leaves of nodes. The problem unavoidably affects the quality of provided anonymity: The availability of anonymity forwarders will be decreased, which reduces the anonymity set; and the frequency of path reformation will increase, which increases the chance of successful intersection attacks. We propose an incentive-based P2P mechanism as an approach to providing reliable anonymity forwarding. It uses incentives to induce the peer nodes to provide anonymity forwarding as reliable service and to make stable and distributed forwarding decisions to minimize the frequency of path reformations. To support incentive, a payment system has been designed which meet the anonymity requirement and can handle typical scenarios of cheating and malicious attacks. To make sound forwarding decisions, we use game theory to carefully design the forwarding strategies used by the peer nodes. We have used event-driven simulations to evaluate the quality of anonymity provided by the mechanism under high churn and with the presence of malicious nodes. The results show that the quality of anonymity is maintained in those scenarios.