Adding 1 teaspoon of ghee or oil to the dough while kneading adds some fat to the dough, which keeps them softer for longer. Also, knead the dough using warm water. Another trick is to store them in a tortilla box or wrap them in aluminum foil as soon as they are made, since exposure to air dries them out. A final trick is to apply 1/2 teaspoon of ghee to the roti while stacking and storing.