The thermal resistance includes the resistance thermal interface material itself and thermal contact resistance, there are three main ways for reduce thermal resistance on thermal interface material itself, which includes increase the effective contact area, increase thermal conductivity, reduce the thickness of the thermal interface materials, here we recommend to choose AOK thermal pad for your application.
The critical influences of the thermal contact resistance are hardness and gap filling performance of the thermal product, the main way to reduce the thermal contact resistance, which are increasing pressure and using softer thermal interface materials.
As mentioned above is AOK thermal pad manufacturer that recommend three ways to reduce the thermal resistance, as for your reference.