Flash mobs should begin from nothing and end with nothing.   A key characteristic of a flash mob is a surprise start.  Mobbers should be encouraged to look as inconspicuous as possible before the mob as to not give the surprise away.  

Once the mob begins, mobbers should get into character or begin the action as quickly as possible.  At this point the mob is only controlled by the mob.  They way people act is based on how the mobbers interpret the original instructions.

When the mob ends, it is very important that the area look and sounds as if nothing had ever happened.  Bystanders generally enjoy this aspect of flash mobs and leave them with smiles on their faces and asking themselves what just happened?