even if not exactly WSN, maybe some ideas could come from this paper we wrote about data forwarding in vehicular sensor networks towards road side units (acting as gateways): "IEEE 802.11p for Cellular Offloading in Vehicular Sensor Networks", https://drive.google.com/file/d/0B9p2h9CFpXT7a0lXWWRtc3dYU2c/view
you may apply the formula used by Leach proto for the election of Cluster Head nodes except it's appliance will be on the subset of nodes that have enough energy and less activity (may be)