In next-generation mobile networks (NGWNs), seamless service for users and handoffs between heterogeneous networks must be taken into account. In this paper, we propose a seamless proxy-based handoff scheme using the PMIPv6-based proxy model, which is able to improve the performance of handoffs in sensor-based mobile networks. Our scheme can efficiently support IP-based seamless mobility by reducing the search process. The performance results show that our proposed scheme outperforms the existing schemes in terms of Quality of Service (QoS), including such factors as throughput, handoff latency, packet loss, and signaling overhead.