Preheat oven to 150 degrees. Slice tomatoes in half lengthwise and place cut side up on cookie sheets. Mix salt and sugar together and sprinkle a little bit over each tomato, or about 1/2 teaspoon. Place in oven and cook slowly for 10 hours. Best to put tomatoes in right before you go to bed and remove them m the morning. Allow to cool and refrigerate for later use.