Many of the most flavorful sauces in any cook's repertoire are made using cooking juices from the main ingredient. This might be as simple as a gravy to go with a roast, or as elegant as an acidic gastrique to spoon over a slice of seared foie gras, but the principle is the same. In most cases the sauce will need to be degreased, as the juices will contain an undesirable quantity of fat.
Pour away the pan juices or finished sauce into a measuring cup or cylindrical container. Let the juices rest for five minutes, after which the fat will be concentrated at the top.
Ladle or spoon the layer of fat away from the sauce. When the fat has been mostly removed, and only a small amount remains on the surface, blot up what's left with paper towel or brown paper, such as strips torn from a paper bag.
Refrigerate the sauce. Most kinds of fat and grease will solidify or congeal on the surface of the sauce, and can be removed more easily. Your freezer will do the same thing in a shorter time, but the sauce must be removed before it freezes.
Degrease the sauce at the table, if necessary, by using a ladle or sauce boat with a built-in skimmer, which holds back the fat floating on top but lets the sauce flow through underneath.