Cooling is a fundamental need as well as a significant energy consumer in a plethora of practically important applications. In this paper, we analyze latent-heat storage using phase-change materials (PCM) as a means for improving temperature control and energy management in cooling systems. We propose a novel, systems-centric approach to PCM-based thermal management and establish a connection between the quantity and geometric properties of the PCM, the dynamics of the integrated system, and potential energy savings. We show that the melting/solidification cycles of PCM provide a thermal buffer effect which can be relied upon to balance the use of passive and active cooling, reducing energy consumption. Subsequently, we focus on composite heat sinks consisting of PCM elements encapsulated in a conductive matrix material as a practical implementation of PCM-enhanced thermal management. Relying on concepts from nonlinear system identification and dynamic optimization, we formulate a novel stochastic optimization framework for selecting the optimal size and size distribution of the PCM elements for minimizing energy consumption under fluctuating loads. Finally, we illustrate our results with a case study.