Opportunistic forward techniques can improve the energy efficiency of wireless multi-hop networks by exploiting multiuser diversity. Opportunistic forward schemes generally integrate the functions of routing and MAC layers; nevertheless, most of the works focus on the design of routing protocol while assuming an energy efficient MAC layer. However, due to the unreliable links among the source node and its relay candidates, a great amount of energy expenditure results from the synchronization and multi-relay transmission, which degrades the energy performance of opportunistic forward techniques. In this paper, an energy efficient opportunistic MAC protocol with the mechanisms of reservation and a relay candidate coordination is proposed. Moreover, the multi-relay transmission probability is analyzed. Simulation and experiment results on a real wireless sensor network platform in different channels demonstrate the the proposed scheme greatly reduces the multi-relay transmission probability and achieves about 84% improvement of energy efficiency compared with the traditional opportunistic MAC schemes.