Energy efficiency is a major design issue in the context of Wireless Sensor Networks (WSN). If the acquired data is to be sent to a far-away base station, collaborative beamforming performed by the sensors may help to distribute the communication load among the nodes and to reduce fast battery depletion. However, collaborative beamforming techniques are far from optimality and in many cases we might be wastingmore power than required. We consider the issue of energy efficiency in beamforming applications. Using a convex optimization framework, we propose the design of a virtual beamformer that maximizes the network lifetime while satisfying a pre-specified Quality of Service (QoS) requirement. We derive both centralized and distributed algorithms for the solution of the problem using convex optimization and consensus algorithms. In order to account for other sources of battery depletion different from that of communications beamforming, we consider an additional random energy term in the consumption model. The formulation then switches to a probabilistic design that generalizes the deterministic case. Conditions under which the general problem is convex are also provided.