The efficient use of energy in wireless sensor networks is a critical issue as the sensor nodes typically operate with limited power sources. Most consumption energy of the sensor nodes is used for communication parts. MAC protocols directly control the communications in network; therefore, in this paper we design an innovative contention-based MAC protocol for wireless sensor networks to provide long network lifetime. Our MAC protocol embeds some extra information in long wake up preamble frame and it also uses collision avoidance signaling and handshaking. These ideas help wireless nodes to stay at sleep mode as much as possible. This protocol avoids major energy waste causes, such as idle listening, collision and overhearing, without compromising other network performance measures like latency. Our simulation results and analyses show that with dynamic traffic load, ET-MAC protocol achieves significant improvement in energy-efficiency and throughput, over well known MAC protocols like B-MAC.