My mom always said Flour = Cold; Cornstarch = Hot. That means, you use flour at the beginning of the recipe in melted butter or sprinkled on potatoes, apples, etc. As it gets hot, it then starts to thicken the liquids. Cornstarch, on the other hand, was mixed into cold water and once the recipe was boiling, the mixture was slowly poured in. This was a general rule, not to be followed religiously.

In the last few years, however, I have discovered a new trick that works wonderfully for thickening chili* and spaghetti sauce**: Cornflour (or Polenta or cornmeal, depending on where you are from). If your spaghetti sauce has turned out too soupy, toss in a couple tablespoons of cornmeal and it soaks the liquid right up, giving the dish a nice, thick consistency without any change in flavor.