// new class extend Ajax.Request
NoCacheAjaxRequest = Class.create(Ajax.Request, {
    // ctor
    initialize: function($super, url, options) {
      // No cache request headers
      noCacheOption = {
        requestHeaders: ['If-Modified-Since', 'Thu, 1 Jan 1970 00:00:00 GMT',
                         'Cache-Control', 'no-cache']
      };
      
      // append the noCacheOption to options
      Object.extend(options, noCacheOption);
      $super(url, options); // Ajax.Request's ctor
    }
  }
);