/* Note:
 * Notice that both _instance & getInstance() are static.  Thus, _instance exists before
 *   any objects of the class have been created, and getInstance() can be called for the 
 *   class and so it can be called before there are any objects.
*/
package mvc
{
	import mx.collections.ArrayCollection;

	/* Because ActionScript requires that all constructors are public, you can't prevent
	 *   anyone from making more Model Objects.  Even without an explicit constructor, it
	 *   would appear as if it can't happen but unfortunately, but the compiler will still 
	 *   synthesisize a default public constructor.  Thus singleton behavior can only happen
	 *   by convention and the best you can do is communicate the intent of the pattern
	*/
	public class Model
	{
		private static var _instance:Model;
		
		[Bindable]
		public var buddies:ArrayCollection = new ArrayCollection( [ "ChrisAIV", "Jimmy", "Sandy", "Frosie" ] );
		
		public static function getInstance():Model
		{
			if( !_instance ) _instance = new Model();
			return _instance;
		}
	}
}

/* Note:
 * Notice that both _instance & getInstance() are static.  Thus, _instance exists before
 *   any objects of the class have been created, and getInstance() can be called for the 
 *   class and so it can be called before there are any objects.
*/
package mvc
{
	import mx.collections.ArrayCollection;

	/* Because ActionScript requires that all constructors are public, you can't prevent
	 *   anyone from making more Model Objects.  Even without an explicit constructor, it
	 *   would appear as if it can't happen but unfortunately, but the compiler will still 
	 *   synthesisize a default public constructor.  Thus singleton behavior can only happen
	 *   by convention and the best you can do is communicate the intent of the pattern
	*/
	public class Model
	{
		private static var _instance:Model;
		
		[Bindable]
		public var buddies:ArrayCollection = new ArrayCollection( [ "Sean", "Sam", "Seth", "Sally" ] );
		
		public static function getInstance():Model
		{
			if( !_instance ) _instance = new Model();
			return _instance;
		}
	}
}

/* Note:
 * Notice that both _instance & getInstance() are static.  Thus, _instance exists before
 *   any objects of the class have been created, and getInstance() canbe called for the 
 *   class and so it can be called before there are any obejects.
*/
package mvc
{
	import mx.collections.ArrayCollection;

	/* Because ActionScript requires that all constructors are public, you can't prevent
	 *   anyone from making more Model Objects.  Event without an explicit constructor you
	 *   would think that it can't happen but unfortunately, but the compiler will still 
	 *   synthesisize a default public constructor.  Thus singleton behavior can only happen
	 *   by convention and the best you can do is communicate the intent of the pattern
	*/
	public class Model
	{
		private static var _instance:Model;
		
		[Bindable]
		public var buddies:ArrayCollection = new ArrayCollection( [ "Sean", "Sam", "Seth", "Sally" ] );
		
		public static function getInstance():Model
		{
			if( !_instance ) _instance = new Model();
			return _instance;
		}
	}
}

Apparently Singletons in AS3 is pretty controversial so rather than provide the various solutions out there, my intent was to explain why it's such a headache.

AS3: Singleton Explanation

ActionScript 3