An analysis has been completed to provide guidelines for producing planar microwave resonators with Q-factors of the order of 10⁶-10⁸ at a temperature of 4 K and a frequency of 1-20 GHz using high-T_c superconducting thin films on sapphire shielded by suitable buffer layers. Buffer layers not only overcome the problems of film-substrate interactions and reduction of microcracks, but can also confine the field into the sapphire which enhances the Q_r and Q_c of the resonator.