Clustering proves to be an effective approach to reduce energy consumption of micro-sensors and prolong the lifetime of wireless sensor networks. In this paper, we address the sensor clustering problem in the design of an autonomous MANET (aMANET) for energy-efficient data collection in wireless sensor networks. In the aMANET context, we formulate the network energy optimization as a weighted K-means clustering problem, and present a distributed technique called MADSEC (Mobility-Assisted Distributed SEnsor Clustering) for sensor clustering and data collection. MADSEC allows the mobile nodes to dynamically run a distributed clustering algorithm to reposition themselves and organize energy-efficient sensor clusters. Extensive NS-2 simulation results indicate that the proposed distributed clustering strategy is as effective as a theoretical centralized K-means clustering technique. Furthermore, MADSEC can at least double the network lifetime of LEACH, a well-known static clustering method.