The loss tangent measurement surface (LTMS), a measurement technique which leverages highly resonant metasurfaces to augment free-space material characterization measurements and enhance sensitivity for characterizing low-loss samples, is presented. The relationship of a metasurface resonance to the varying permittivity and thickness of arbitrary dielectric layers on either side of the metasurface is explored, yielding an effective permittivity model to be used in conjunction with equivalent circuit modeling (ECM) techniques for rapid and fully predictive computation of LTMS behavior. General trends and strategies for LTMS design are presented, followed by a specific realization of the LTMS based on the asymmetric split-ring resonator (aSRR), which is used to obtain the complex permittivity of two different materials. Other topics tangentially related to the LTMS are presented; a reconfigurable dogbone metasurface incorporating varactor diodes to change the resonant frequency is designed using hybrid full-wave / circuit simulations and ECM techniques, showing good model-measurement agreement. Global optimization of metasurfaces using the novel periodic level set function (p-LSF) is shown to be promising for engineering novel metasurface and planar antenna array elements.