Localization accuracy and energy efficiency are two important goals in sensor networks. In this paper, we propose a new medium access control (MAC) scheme to prioritize nodes with higher location estimation accuracy (location reliability) and higher residual energy level. The proposed solution is to encode the information of location reliability and residual energy level with a decreasing backoff function. All the nodes are divided into several classes according to their location reliability and residual energy level. Using the proposed decreasing backoff function, different classes have different backoff windows. The higher priority classes have higher probability to access the network. Theoretical analysis and simulation results show an improved performance for the proposed MAC solution compared to the traditional exponential backoff scheme.