Use canned frosting to make decorating icing. Just add 1TBLSP - 1/2c confectioners sugar per can & mix well with mixer. (Will expand) Try a little at a time. If too thick add water. Can also add drops of food coloring or 8oz of soft cream cheese.
When you buy a container of cake frosting, whip it with your mixer for a few minutes. The air will double it in size, so you can frost more cake (or cupcakes) with the same amount :) Oh - Also cuts down on the sugar and calories per serving ;)