Network lifetime maximization is an important problem particularly for large-scale wireless sensor networks. To this end, we take an approach that exploits the 'sink mobility'. The sensor nodes near the sink node tend to suffer high energy consumption due to heavy traffic relay operations. As the sink node moves around, such energy bottleneck can be alleviated. We analyze the optimal sink mobility pattern obtained by solving a linear programming model and use the insights from the analysis to design a simple practical heuristic. Our heuristic utilizes only local information, e.g., the standard deviation of residual energy of neighboring sensor nodes. The simulation results show that our scheme achieves near-optimal performance even with the slowly moving mobile sink.