We consider a wireless sensor network where neighboring sensor nodes are organized into groups to form coalitions and data from one coalition are relayed through multiple hops to reach the sink. We assume that data aggregation and cooperative communications can be carried out within each coalition. We focus on energy balancing across sensor nodes and investigate both intra-coalition and intercoalition energy balancing. More specifically, for intracoalition energy balancing, we propose schemes where one or more sensor nodes in a coalition, not necessarily the coalition head, are selected to transmit the data, with additional achievement of multiuser diversity or cooperative diversity. Building on the coalition-aided data transmission schemes, we study the optimal coalition planning, via using unequal coalition sizes based on sensor locations, to balance the energy consumption across different coalitions. In particular, we examine two types of network models with different traffic patterns: in a Type I network, only part of the sensor nodes have data to transmit and other sensors serve as relays; and in a Type II network, all sensor nodes have data to transmit. Comparing with traditional clustering schemes, we show that better energy balancing across sensor nodes can be achieved with the proposed coalition architecture and that network lifetime can be prolonged significantly.