This paper addresses a systematic procedure of placing actuators and sensors to identify modal parameters of flexible structures. In this procedure, using a pre-test analysis model, actuator locations are selected to minimize the condition number of the controllability matrix and sensors are placed to obtain a full-rank Hankel matrix with the minimum condition number. Since the rank of the Hankel matrix determines the number of modes identified by a modal parameter identification algorithm such as the Eigensystem Realization Algorithm, modal identification of the target modes for a modal testing can be ensured. A suboptimal method, called the Effective Independence approach, is employed to select the best locations among the candidate actuator/sensor locations. In this approach, the candidate actuator and sensor locations contributing little to the linear independence of the controllability matrix and the Hankel matrix, respectively, are removed iteratively until a desired number of actuator/sensor locations remains. Unlike computationally intensive search techniques, this approach is computationally efficient. An example using a ten-bay truss structure is presented to illustrate the design procedure.