Best way to cook duck so its tender
Make sure the skin is dried (2 to 3 days in the refrigerator), so it comes out crispy, not soft. The fat has to be rendered when cooking without drying out the meat (a roaster with a rack and an hour before finish cooking, remove the fat, put the duck back in the roaster, pierce the skin and roast and finish roasting). The flavour will be enhanced with just the right amount of seasoning, but don't overwhelm it (sea salt, marjoram, garlic powder, and an apple).
Assuming that you are roasting a whole duck, begin with the assumption that a huge portion of its body weight is fat. I would first rub the outside with sea salt and be prepared to baste at regular intervals from the bottom of the roasting pan as it cooks and the fat melts out. This will give you a crispy skin and moist meat, but mind your oven temperature, you don't want to burn the damn thing. There are plenty of herbs and spices that complement duck, but consider also what your sides are going to be.
I rarely cook whole ducks anymore, because the breasts are great when they're seared over a hot grill, and the legs aren't tender unless they get the opposite treatment. For them, I salt and pepper them and brush them with a thin coat of Dijon mustard. Then I roast them at 325 degrees for 1 and a half to 2 hours. It's the long slow roasting that's the key to tenderness.