After 45 minutes, turn the duck over. Add the sugar, soy sauce and salt. Continue simmering for another 45 minutes. Turn duck once again and simmer until tender and almost falling apart, about another 45 minutes. Turn off the heat and when cool enough, carefully remove duck from pot and place in a roasting pan. Cover and refrigerate until the next day.
Be very gentle with the duck, it will be close to, if not, falling apart.