Ducks carry all their fat right under their skin. The skin and fat trim can be rendered into a smooth very creamy and delicious fat. This fat is great for all sort of culinary uses. Cooking the skin and fat at low to medium heat until the skin is crispy and golden, straining the solids from the fat and storing in the refrigerator for around 4 months.
Alternately, you could make the fat solid in the fridge and the cut into
smaller portions and freeze in an airtight bag for quick use later on.
You can also use the fat that renders from roasting a whole duck in the
same way. There is some debate for how long you can keep fats. If you do
not scorch the fat, it can be strained and reused. When reusing fats,
if it is not clear and gives off an offensive odor, it has probably been
scorched and should not be used. Your nose will always be the surest
way to keep from eating or using rancid foods.