Here is the solution that I use in my editor scripts:

static class CustomGUI {

    public static readonly GUIStyle splitter;

    static CustomGUI() {
        GUISkin skin = GUI.skin;

        splitter = new GUIStyle();
        splitter.normal.background = EditorGUIUtility.whiteTexture;
        splitter.stretchWidth = true;
        splitter.margin = new RectOffset(0, 0, 7, 7);
    }

    private static readonly Color splitterColor = EditorGUIUtility.isProSkin ? new Color(0.157f, 0.157f, 0.157f) : new Color(0.5f, 0.5f, 0.5f);

    // GUILayout Style
    public static void Splitter(Color rgb, float thickness = 1) {
        Rect position = GUILayoutUtility.GetRect(GUIContent.none, splitter, GUILayout.Height(thickness));

        if (Event.current.type == EventType.Repaint) {
            Color restoreColor = GUI.color;
            GUI.color = rgb;
            splitter.Draw(position, false, false, false, false);
            GUI.color = restoreColor;
        }
    }

    public static void Splitter(float thickness, GUIStyle splitterStyle) {
        Rect position = GUILayoutUtility.GetRect(GUIContent.none, splitterStyle, GUILayout.Height(thickness));

        if (Event.current.type == EventType.Repaint) {
            Color restoreColor = GUI.color;
            GUI.color = splitterColor;
            splitterStyle.Draw(position, false, false, false, false);
            GUI.color = restoreColor;
        }
    }

    public static void Splitter(float thickness = 1) {
        Splitter(thickness, splitter);
    }

    // GUI Style
    public static void Splitter(Rect position) {
        if (Event.current.type == EventType.Repaint) {
            Color restoreColor = GUI.color;
            GUI.color = splitterColor;
            splitter.Draw(position, false, false, false, false);
            GUI.color = restoreColor;
        }
    }

}

http://forum.unity3d.com/threads/17066-How-to-draw-a-GUI-2D-quot-line-quot?p=230386#post230386