We present PAN-on-Demand, a self-organizing wireless personal-area network (PAN) that balances performance and energy concerns by scaling the structure of the network to match the demands of applications. PAN-on-Demand autonomously organizes co-located mobile devices with one or more commodity radios such as Bluetooth and WiFi to form a network that enables data sharing among those devices. It improves performance and extends battery life by switching between interfaces and opportunistically exploiting available power-saving modes. When applications are actively using the network, PAN-on-Demand offers high-bandwidth, low-latency communication; when demand is light, it adapts the network structure to minimize energy usage. Our results show that PAN-on-Demand reduces the average response time of PAN applications such as MP3 playing, e-mail viewing and photo sharing by up to 92% and extends battery life by up to 47% compared to current PAN communication strategies.