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Summary form only given. In hierarchical two-tiered sensor networks, higher-powered relay nodes have recently been proposed to be used as cluster heads for designing scalable sensor networks.The assignment of sensor nodes to clusters in an energy-efficient way is known to improve the lifetime of such networks. In this paper we have proposed two efficient distributed algorithms for assigning sensor nodes to clusters in two-tiered networks. The first heuristic assumes that all relay nodes, acting as cluster heads, send their data directly to the base station. The second heuristic relaxes this assumption and is to be used with any network where each relay node uses a multi-hop route to send its data to the base station. Simulations on networks of different sizes show that our approaches consistently outperform existing heuristics for clustering in two-tier sensor networks and are fast enough to be used for practical networks containing hundreds of sensor nodes. We have compared the results of our distributed approaches with the optimal solutions obtained using an existing approach based on an integer linear program (ILP) formulation and have shown that, on an average, our approaches, with a relatively small overhead, can produce results that are close to the optimal solutions.