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Most peer-to-peer (P2P) systems assume that all peers are cooperating for the benefit of the community. However in practice, there is a significant portion of peers who leech resources from the system without contributing any in return. We propose a simple selfish link-based incentive (SLIC) mechanism for unstructured P2P file sharing systems to create an incentive structure where in exchange for better service, peers are encouraged to share more data, give more capacity to handle other peers' queries, and establish more connections to improve the P2P overlay network. Our SLIC algorithm does not require nodes to be altruistic and does not rely on third parties to provide accurate information about other peers. We demonstrate, through simulation, that SLIC's locally selfish and greedy approach is sufficient for the system to evolve into a "good" state.