This article addresses the coalition formation problem in a multi-agent context where agents are self-interested and plan their activities dynamically. When agents are self-interested and free to decide which strategies they use, we cannot expect them to form coalitions with specific properties. We develop a new coalition formation model that uses the plans of the agents to guide the search for the coalitions and allows to analyze the coalition proposals already suggested by other agents in order to derive their preferences. This eases the negotiation for the coalitions. We present in this paper our coalition formation mechanism.