Wireless sensor networks are becoming increasingly important in recent years due to their ability to detect and convey real-time information for many civilian and military applications. Wireless sensor network deployed thousands of nodes in not easily accessible places that operate with batteries. It can be difficult or impossible to replace or recharge the batteries of the sensor nodes. So to maximize the network lifetime has become a big challenge in the wireless network. This paper presents a brief survey on various techniques employed for maximizing network lifetime of a wireless sensor networks. The authors also address some other challenges related to network lifetime of wireless sensor networks that need to be considered in future designs.