It was Les Besser, I think, who I heard say "Impedance matching is the bread and butter of the microwave engineer."    Impedance matching has been described as having two features:  parasitic absorption and resistance level transformation.  A very good first technique to know about is 3-step approach found in a 1988 Hewlett Packard Application Note for the Butterfly Mixer, which has been available  here.  

The 3-steps can be described as:  Resonate, Wrap, and Transform.  The slides below illustrate the approach using the example from App Note 976 for the Butterfly Mixer.