Whole whipping cream
grated parmesean cheese
(optional) any additional cheese
Heat the cream on low to high slowly stirring in other ingredients untill cheese is melted and desired consistancy is reached. Should the sauce be too watery no matter how much cheese is added (a general rule of thumb is to add slightly more cheese in volume than cream, although I usually add a bit extra) you can add a bit of plain white flour to thicken it up.