This is what I have learned by scouting around on the net......
Royal Icing Tips
• Store icing in a glass or metal container; it will break down in plastic.
• Reserve a set of pastry bags, couplers, and tips exclusively for royal icing. (Your old cake-decorating bags may not be grease-free.) Disposable pastry bags or parchment triangles rolled into cones will work.
• Store filled pastry bag tip-down in mug with damp paper towels in bottom. Seal top with twist-tie or rubber band, and drape with plastic wrap.
• Keep bowl of icing covered with damp paper towels and plastic wrap at all times. Uncovered, it will harden like concrete.
from: Food - Royal Icing - CottageLiving.com
Make sure your bowls and mixing equipment is grease free!
Be sure to sift your powdered icing sugar.
Add icing sugar slowly...1 tablespoon,or so, at a time.
Use a recipe with Cream of Tartar, to insure a smooth consistency.
Add a bit of glycerin if you don't want the "rock-hard" consistency.
Make sure your cake is completely cold!
Do not refrigerate (unused icing - nor the iced cake ), cold causes weeping.
Do not store in any plastic containers.YouTube - Episode 2 - How to make Royal Icing