One of the most critical issues in wireless ad hoc networks is represented by the limited availability of energy within network nodes. The time period from the instant when the network starts functioning to the instant when the first network node runs out of energy, the so-called network life- time, strictly depends on the system energy efficiency. Our objective is to devise techniques to maximize the network life-time in the case of cluster-based systems, which represent a significant sub-set of ad hoc networks. We propose an original approach to maximize the network life-time by determining the optimal clusters size and the optimal assignment of nodes to cluster-heads. The presented solution greatly outperforms the standard assignment of nodes to cluster-heads, based on the minimum distance criterion.