Generally, you clean the top surface of the contacts on the board, and the underside of the "Arm" contact.
Easiest way to clean the arm, is to slip a piece of 1000grit Wet 'n' Dry under the arm with the sanding side facing the arm and gently slide back and forth between the two.
Finish by cleanin with a soft lint free cloth in the same way.
Personally, i would go with an ESC, it cuts out all that hassle of keep cleaning the Mech SC and is much smoother on power delivery to the motor, but then, thats what I did with my M03 anyway, I will not run a mech SC if I can avoid it.