Litz wire planar spiral coils are widely used in inductive power transfer systems due to low power loss in high-frequency operation and low profile. However, the complicated structure of the litz wire coil imposes difficulties in estimating the frequency-dependent resistance and quality factor accurately. In this work, we present an 2D analytical model for calculating the frequency-dependent resistance and quality factor of multi-coil inductive power transfer systems based on superposition of different loss effects. Its accuracy is validated with multiple coils and litz wires over wide frequency range. Meanwhile, a fast and accurate multi-objective optimization method is developed to improve the product quality factor and coupling factor. From the optimization results, there are five factors affecting the quality factor and coupling factor. A set of design guidelines is proposed to cope with these design factors. The final prototypes show 33\% increase in quality factor and 23\% improvement in coupling factor compared with the state-of-the-art designs. Meanwhile, a 30W, 500kHz inductive power transfer system is designed with 91\% peak efficiency.