function buildId(id) {
  return 'data[Experience][' + id + '][experience_content]';
}

jQuery('#foo').click((function () {
  var i = 0;
  return function () {
    var $target = jQuery(buildId(i));
    $target
      .clone()
      .attr('id', buildId(i + 1))
      .insertAfter($target);
    i = i + 1;
  };
})());