In a small bowl, mix together the salt and sugar. Put the pork in a large, shallow bowl and rub the salt and sugar mixture all over it. Cover with clingfilm and refrigerate for at least 6 hours, or overnight.
An hour before you plan to start cooking, remove the pork from the fridge and discard any juices. Preheat the oven to 150C/300F/gas mark 2.
Put the pork in a roasting pan and into the oven. After the first hour, baste with pan juices and repeat every hour. Cook for about 6 hours, or until it collapses, yielding easily to the tines of a fork.
Let the meat cool for 15 minutes, then shred it using two forks. Taste and adjust the salt as needed. Serve with sour cream, tortillas and slaw.