Purchase incrementally as the cost goes down, making a normal position and planning to purchase more as the value diminishes further. Hold up until the point when the value settles, and maybe even shows sign of recovering, and purchase at that point. Set purchase orders at bring down costs than the present cost and let them fill. Setting purchases just before notable help levels, vast "buy walls," and mental levels is a particularly good strategy (as costs have a tendency to do at least a quick bounce off these levels).