Skeleton of jQuery plugin

jQuery Plugin Factory

;(function($, undefined) {
  var PLUGIN_NAME = 'plugin';
  var DEFAULT_OPTIONS = {
    optionA: true,
    optionB: false,
    'callbackC': function() {
    }
  };

  function plugin(options) {
    var element = this;
    var $element = $(element);
    var instance = {};
    var options = $.extend({}, DEFAULT_OPTIONS, options);

    // main action here:
    // ...

    instance = $.extend(instance, {
      // public methods:
    });
    return instance;
  }

  $.fn[PLUGIN_NAME] = function(method, arg) {
    var args = arguments,
        results = undefined, $selector;

    $selector = $(this).each(function() {
      var $this = $(this),
          instance;

      instance = $this.data(PLUGIN_NAME);
      if (instance === undefined) {
        if (typeof method !== 'string') {
          $this.data(PLUGIN_NAME);
          instance = plugin.call(this, method);
          $this.data(PLUGIN_NAME, instance);
        } else {
          $.error('Function "' + method + '" is called before "' + PLUGIN_NAME + '" is initialized.');
        }
      } else {
        if (method in instance && $.isFunction(instance[method])) {
          results = instance[method].apply(instance, Array.prototype.slice.call(args, 1));
        } else {
          $.error('Function "' + method + '" is not found in "' + PLUGIN_NAME + '".');
        }
      }
    });
    return results !== undefined? results: $selector;
  };
}($));

Optional css file loading:

    var src = $("head script").last().attr("src") || '';
    var scriptpath = src.split('?')[0].split('/').slice(0, -1).join('/')+'/';
    var csspath = scriptpath + PLUGIN_NAME;
    var link = $('<link href="' + csspath + '" rel="stylesheet">');
    $('head').append($(link));