To make a simple, basic turkey brine, combine 2 gallons of cold water with 2 cups kosher salt and 1 cup granulated sugar in a large food-safe container, mixing the ingredients until the salt and sugar dissolve. Submerge the turkey, and let it brine for 18 to 24 hours.
For a more flavor-filled brine, combine 1 quart of water with 1 1/2 cups coarse salt, 6 bay leaves, 2 tablespoons whole coriander seeds and 1 tablespoon dried juniper berries in a saucepan. Add 2 tablespoons whole peppercorns, 1 tablespoon fennel seeds and 1 teaspoon mustard seeds, and bring the mixture to a simmer. Stir the solution until the salt dissolves.
Meanwhile, line a 5-gallon container with a brining bag. Place an 18- to 20-pound turkey in the bag along with the cooled salt and herb mixture and 24 cups water. Stir in thinly sliced onions, fresh thyme and crushed garlic cloves. Tie the bag, and refrigerate the turkey for 24 hours.
Alternatively, prepare a dry brine for the turkey by adding 10 sprigs of thyme, 6 sprigs of sage and 4 sprigs of rosemary in a food processor. Add 4 cloves garlic, 2 teaspoons celery seeds and 1/2 teaspoon black pepper. Stir in 1/2 cup kosher salt and 2 tablespoons granulated sugar to the mixture. Rub the dry brine over the turkey's skin, and refrigerate it uncovered for six to seven hours.