The successful deployment of peer-to-peer (P2P) live video streaming systems has practically demonstrated that it can scale to reliably support a large population of peers. However, peers, representing rational end users, tend to be non-cooperative when it comes to the duty rather than the self-interests, running counter to the fundamental design philosophy of P2P concept. The objective of this paper is to investigate the problem of encouraging users to balance what they take from the system with what they contribute. We first make a statistical analysis to the service logs of a practical P2P live video streaming system and reveal intrinsic characteristic of users' online duration. Second, we thus propose a novel incentive mechanism based on the composite contributions which consist of two objective metrics, i.e. the online duration and effective upstream traffic. This mechanism offers service differentiation to users with different contributions and has some desirable properties: (1) distributed nature upon gossip-based overlay structure and (2) practical oriented evaluation criteria. The experiment results over PlanetLab further verify its effectiveness.