When rolling out the cookie dough, make sure it is chilled (not frozen), for the dough to roll easily.
Rolling the dough between cling film is the easiest way to ensure evenly and smoothly rolled dough
Royal icing is best made with egg whites, always use organic, and if possible pasteurized egg whites. Meringue powder is a good substitute. Here is a good guide to preparing royal icing and storing it: http://sweetopia.net/2011/01/how-do-you-store-royal-icing-and-how-long-can-you-keep-it/