To prevent cake from drying out, don't over mix it or over bake it. Make sure it is covered completely with an air tight seal when storing. Additionally, keep in mind that sugar does much more for a cake (or any baked good) than just adding sweetness. Baking is a science and sugar helps keep baked goods moist, since it absorbs moisture. Because of this, if you try to alter the sugar in recipes, you will inadvertently alter the moisture content as well. This cake keeps it's moisture fairly well. However, if you're in doubt, you can also keep cakes moist by drizzling them with simple syrup.