Coalition formation is a major research issue in multiagent systems in which the agents are self-interested. In these systems, agents have to form groups in order to achieve common goals, which they are not able to achieve individually. A coalition formation mechanism requires two definition levels: firstly agents need a common protocol to reach an agreement and secondly individual strategies are required to make efficient proposals. Both issues are addressed in this paper. First, we propose a two-phase decentralized protocol that allows agents to interact directly through message passing. Secondly we propose some strategies which allow agents to make clever proposals using the information that has already been collected from other agents. The experimental evaluation shows that the proposed mechanism allows agents to efficiently form coalitions and that the strategies make real improvements for the coalition search process.