Ultra-wideband (UWB) communications has emerged as a burgeoning technology for high data rate wireless personal area networks (WPANs). In this paper, we propose a novel resource utilization mechanism (RUM) for improving the throughput in multi-rate UWB-based WPANs. RUM is intended to remedy a critical issue in both unicast and multicast transmissions. In unicast (single- and multi-hop), the connectivity of a source-destination pair is defined by the ability to overhear control messages (e.g., route requests, request-to-send/ clear-tosend, etc.). These messages are usually sent at a low transmission rate to extend their reachability, hence a node can directly communicate with faraway destinations. Such destinations cannot be reliably reached by high transmission rates. This leads to a long channel reservation time and hence a high blocking probability for prospective reservations and low network throughput. In the case of multicast, the maximum transmission rate is bottlenecked by the farthest destination. RUM exploits opportunistic-relaying and time-spreading techniques to improve link reliability and increase the transmission rate, and hence network throughput. Simulations are used to demonstrate the performance gain of RUM.