Replacing motherboard. Removed CPU fan and see that there is residue from the thermal pad on the CPU and the fan mating surfaces. Before I mess up the nice flat surfaces what is the best technique to remove the old thermal pad stuff?

Use a soft lint free cotton material (old t-shirt), if you plan on using the CPU chip in the future find a static free surface when removing the thermal compound. I have a old beat up mouse pad that is used as a static free surface. Also if the thermal compound gets on your fingers and clothing, it is difficult to remove.

I use, as said, soething lint free and soft, and wipe it off with rubbing alcohol. Just don't forget to put more on before reinstalling!

In addition to alcohol and a lint free rag you can always try ArctiClean by Artic Silver.

Used it recently and was very happy with the results.

ArctiClean

Also most thermal compounds should come off your skin easily with dish soap.