This experiment tested whether changing one's nonverbal behavior prior to an important social evaluation could improve performance on the evaluated task. Participants adopted expansive, open (high-power) poses or contractive, closed (low-power) poses, and then prepared and delivered a speech to two evaluators as part of a mock job interview -- a prototypical social evaluation. All speeches were videotaped and coded for overall performance and hireability as well as for two potential mediators: speech content (e.g., content, structure) and speaker presence (e.g., captivating, enthusiastic). As predicted, those who prepared with high-power poses performed better and were more likely to be chosen for hire; this relationship was mediated by speaker presence, but not speech content. Power-pose condition had no effect on body posture during the social evaluation, thus revealing a relationship between preparatory nonverbal behavior and subsequent performance, and highlighting preparatory power posing as a simple performance-boosting tool with the potential to benefit almost anyone.