Thanks to the rapid development of wireless communication and microelectronics, wireless sensor networks emerged and have been used in various fields to sense the characteristics of objects. Considering that the power of each node in a wireless sensor network is provided by a battery, and in most cases the battery is difficult or impossible to be replaced, how to effectively save the energy of the node is very important during the network runtime. To resolve the above issues, various energy-saving schemes have been proposed. In this paper, the classification of energy-saving techniques is illustrated firstly. Then, we present the typical protocols to prolong the lifetime of network in each category.