Gradual request for radio frequencies makes the traditional static spectrum allocation unable to cover all users' needs in terms of spectrum resources. Accordingly, it is extremely necessary to introduce dynamic and more efficient spectrum assignment approaches. Cognitive radio was conceived as an innovative technology to enhance spectrum utilization by allowing dynamic and opportunistic access. Previous works in cognitive radio context have more focused on spectrum sensing and sharing concepts than spectrum handoff. However, nodes' mobility still arises as a major issue. Thus, we present in this paper a new approach for handoff management including a spectrum sharing solution and a spectrum handoff decision mechanism. Our proposal relies on multi-agent negotiation to enable cognitive radio terminals switching towards the best available spectrum band, while respecting users' applications requirements and environmental conditions. Simulations results prove that our approach gives four main contributions. First, it performs efficient spectrum sharing allowing spectrum' use to reach up to 90%. Second, it ensures smooth and prompt spectrum handoff. Third, our proposal minimizes the handoff blocking rate which is mandatory to avoid service interruption during users' mobility. Finally, it guarantees a high utility for cognitive radio users.