Chop the garlic cloves coarsely, then combine them with the salt in a small bowl or a mortar. Using the pestle or, in the case of a small bowl, the back of a spoon, crush the garlic and salt together until you have a smooth paste. The garlic should be thoroughly crushed, almost to a cream. Slowly beat in the olive oil, stirring constantly, as if you were making a mayonnaise. Add at least 1/3 cup of oil but if you can do so without the sauce breaking,add as much as 1/2 cup. At the end, gently stir in the yogurt, incorporating it fully into the sauce.