Volunteer forwarding, as an emerging routing idea for large scale, location-aware wireless sensor networks, recently has attracted a significant amount of research attention. However, several critical research issues raised by volunteer forwarding, including priority assignment, acknowledgement collisions and communication voids, have not been well addressed by the existing work. In this paper, we propose a priority-based stateless geo-routing (PSGR) protocol to address these issues. Based on PSGR, sensor nodes are able to locally determine their priority to serve as the next relay node using dynamically estimated network density. This effectively suppresses potential communication collisions without prolonging routing delays. PSGR also overcomes the communication void problem using two alternative stateless schemes, rebroadcast and bypass. We analyze energy consumption and delivery rate of PSGR as functions of transmission range. An extensive performance evaluation has been conducted to compare PSGR with competing protocols, including GeRaf, IGF, GPSR and Flooding. Simulation results show that PSGR exhibits superior performance in terms of energy consumption, routing latency and delivery rate, and soundly outperforms all of the compared protocols.