I allways thought it could be simplyfied too surface to air resistance taking the difference in ambient and surface temprature into account.
Airflow help keeping the ambient temprature lower. Otherwise the air directly around your heatsink will warm up too much reducing the heatflow. Then there is resistance, hence larger surfaces provide less surface to air resistance. Here the material of your heatsink is important too. And added to the surface to air resistance is the object to heatsink resistance. Here the thermal paste comes in.
You want to lead the heat away from the object to somewhere else, usually the air. The heat pumping can be accelerated by means of liquid. But as a true airhead I don't like that.
Like all Beetle drivers say: 'Wasser ist um eier kochen' ;-)
