Combine brown sugar, soy sauce, sherry or apple juice,vegetable oil, ginger, dry mustard and garlic powder, mix well.
Pour 1 cup of the marinade in to a large resealable plastic bag(one large enough for the turkey breast to fit in); put turkey in bag. Seal bag and turn a couple times to coat turkey. Place bag on a tray and refrigerate overnight. Pour the remaining marinade in a container and cover and refrigerate for use in basting turkey.
In the morning remove turkey from bag and place on a rack in a shallow roasting pan. Bake uncovered in a preheated 350 degree oven. Bake for 2 to 2-1/2 hours. Baste every 30 minutes with the reserved marinade. After removing from oven let stand 10 minutes before slicing.