Updating editor GUI on button press

I'm trying to create a layout for an editor aimed to make Element lists in the inspector more user friendly. Right now I'm building a column of ObjectFields from an ArrayList, but I'd like to add/remove items from the ArrayList through the press of a button. My biggest concern right now is updating the editor. I can easily edit the ArrayList, but I can't update the amount of ObjectFields in the inspector. I'm at loss here really.

Here's as far as I've gotten.

 public override void OnInspectorGUI()
     {
         JEWaveManager waveManager = target as JEWaveManager;
 
         ArrayList arr = new ArrayList();
         arr.Add ("Element");
         arr.Add ("Element");
 
         Vector2 scrollPos = new Vector2 (0.0F, 0.0F);
         /*GUILayout.BeginScrollView(scrollPos, GUILayout.Width (100), GUILayout.Height(100));*/
         GUILayout.BeginVertical ();
 
         Object source = null;
         foreach (string s in arr)
             EditorGUILayout.ObjectField(s, source, typeof(Object), true);
 
         if (GUILayout.Button("New Item"))
         {
             arr.Add ("Element");
             Repaint ();
         }
 
         GUILayout.EndVertical ();
         /*GUILayout.EndScrollView ();*/
     }