Using a lint free cloth (such as a coffee filter) and high percentage isopropyl alcohol, clean the bottom of the heatpad on the heat sink. It should have a bright sheen to it once you have finished.

Take care not to bump the thermal sensor (shown in red)

You can also clean the paste from the old processor if you are planning on keeping it- just be very careful not to damage the underside of the CPU where the contacts are.