Main memory in many tera-scale systems requires tens of kilowatts of power. The resulting energy consumption increases system cost and the heat produced reduces reliability. Emergent memory technologies will provide systems the ability to dynamically turn-on (online) and turn-off (offline) memory devices at runtime. This technology, coupled with slack in memory demand, offers the potential for significant energy savings in clusters of servers. However, to realize these energy savings, OS-level memory allocation and management techniques must be modified to minimize the number of active memory devices while satisfying application demands. We propose several page shaping techniques and structural enhancements to proactively and reactively direct allocations to a minimal number of devices. To evaluate these techniques on real systems, we implemented these shaping techniques in the Linux kernel. Experiments using our OS extensions coupled with a simple history-based heuristic (to track demand and control state transitions) yield up to 60% energy savings with less than 1% performance loss for various benchmarks including lmbench and SPEC.