In a time-varying fading channel, opportunistic transmission when the channel is strong is known to yield a significant gain. A similar result appears in the context of multiuser channels where multiuser diversity gain is achieved by serving users with the strongest channels. In contrast, in this paper, we exploit opportunistic channel selection to reduce the processing energy at the receiver side. This saving is derived by increasing the gap between the instantaneous capacity and the code rate, which in turn reduces the required decoding energy. The savings in the processing energy then enables the receiver to sample and process a larger fraction of the packets. We first analyze the single-user case, and propose a signaling scheme that achieves the optimum rate. We then extend this result to a multiuser system.