A fundamental challenge in the design of wireless sensor networks (WSNs) is to maximize their lifetimes. Since many sensors have correlated readings, data aggregation has emerged as an efficient approach to reduce the number of transmissions, and hence minimize overall power consumption in the network. An important aspect of data aggregation is the placement of aggregation points and how aggregated data is routed to the gathering points. In this paper, we consider the problem of correlated data gathering in WSNs with the objective of minimizing the total transmission cost in terms of power consumption. We particularly focus on the problem of finding the set of aggregation points that satisfy our objective. Two solution strategies are presented: an exact solution using an integer linear program (ILP) formulation and a near optimal, but simple and efficient heuristic, called load balancing with aggregation (LISA). Our solutions use a fixed virtual wireless backbone that is built on top of the physical topology. Numerical results show that our proposed scheme provides substantial energy savings when compared to other schemes.