In this work we propose an energy-efficient transmission strategy for wireless sensor networks that operate in a strict energy-constrained environment. Our transmission algorithm consists of two components: a binary-decision based transmission and a channel-aware backoff adjustment. In the binary-decision based transmission, decision on whether to transmit or not is absolutely dependent on the current channel conditions. Specifically, transmission is initiated only when the channel quality exceeds a specified threshold, so that unsuccessful transmissions causing a waste of energy are avoided whenever possible. Using the Markov decision process (MDP) formulation we obtain the optimum threshold for successful transmission. A channel-aware backoff adjustment, the second component of our proposal, is introduced to favor nodes with better channel conditions. By intelligently combining these two ingredients our transmission algorithm attempts to maximize energy efficiency. Extensive simulations are performed to verify the performance of our proposal over fading wireless channels. Numerical results show that our transmission algorithm outperforms the existing approaches in terms of energy efficiency, thereby further prolonging the network lifetime.