This work mainly focuses on the optimization of the control strategy to obtain a bistable NES that achieves maximum efficiency under different harmonic excitations. As excitation increases, four unique regimes appear in turn. According to its distribution of characteristic responses regimes, the maximum efficiency point occurs when the Strongly Modulated Response (SMR) disappears. The optimal stiffness for a given excitation is expressed as a function of negative stiffness, which gives the possibility of controlling it by adjusting the pre-compressed length in a four spring system. The model of the device consists of 2 conical and 2 linear springs that counterbalance one another and provide the desired stiffness. The optimal absorbing range of the device is found by numerical investigation. The result of the primary static experiment proves that the model and stiffness control strategy are valid. This work offers some fundamental procedures for the optimal design of a bistable NES and paves the way for its practical implementation.