Energy costs are a critical consideration for operating High-Performance Computing (HPC) systems, with significant efforts dedicated to reducing the energy expenditure of active computations. However, compute nodes of HPC systems also spend a non-negligible amount of time idling, i.e., without performing any useful work. Optimizing idle power consumption presents an impactful and potentially more accessible opportunity for practical energy savings. This paper surveys factors that influence the idle power consumption of HPC systems. We employed various monitoring tools and power measurements in a structured practical approach to optimize idle power consumption during the installation phase of an HPC system. The results highlight the effectiveness of targeted idle power management strategies, demonstrating compound savings of approximately 90 W per node, and 57 kW for the full system in idle. This thoroughly discussed example can serve as a blueprint for similar optimizations in other HPC environments.