This paper describes a novel, general methodology for designing a parallel, passive-assist device to augment an active system using optimization based on a known maneuver of the active system. Implementation of the passive-assist device can result in an improvement in system performance with respect to efficiency, reliability, and/or utility. The methodology is demonstrated with a torsional spring designed to minimize energy consumption of a prototypical unmanned ground vehicle robot arm. Initial results indicate that this procedure can reduce maximum required torque by and energy consumed by as much as 25%. The proposed method is experimentally verified and compared to other state-of-the-art design approaches.