The big key to royal icing is making sure that your tools, and work area, are free of grease. Since royal icing is a meringue, it begins to break down as soon as it comes into contact with oil. So, if you are using your decorating bags or tips for buttercream, make sure you clean them with vinegar before using them. I also recommend using either a new bag, or making one out of parchment paper, rather than risking using a greasy bag.