_form.html.haml

- form_for @products do |form|
  %h3== Specifications
  %ul#specifications_container
    - form.fields_for :specifications do |specification_form|
      = render :partial => "specification", :locals => { :form => specification_form }

  %p= add_child_link "Add Specification", :specifications

  = new_child_fields_template form, :specifications

  %p= form.submit

application.js

$(function() {
  $('form a.add_child').click(function() {
    var association = $(this).attr('data-association');
    var template = $('#' + association + '_fields_template').html();
    var regexp = new RegExp('new_' + association, 'g');
    var new_id = new Date().getTime();

    $("#" + association + "_container").append(template.replace(regexp, new_id));
    return false;
  });

  $('form a.remove_child').live('click', function() {
    var hidden_field = $(this).prev('input[type=hidden]')[0];
    if(hidden_field) {
      hidden_field.value = '1';
    }
    $(this).parents('.fields').hide();
    return false;
  });
});

application_helper.rb

module ApplicationHelper
  def new_child_fields_template(form_builder, association, options = {})
    options[:object] ||= form_builder.object.class.reflect_on_association(association).klass.new
    options[:partial] ||= association.to_s.singularize
    options[:form_builder_local] ||= :form

    content_for :javascript_templates do
      content_tag :div, :id => "#{association}_fields_template", :style => "display: none" do
        form_builder.fields_for(association, options[:object], :child_index => "new_#{association}") do |f|
          render(:partial => options[:partial], :locals => { options[:form_builder_local] => f })
        end
      end
    end
  end

  def add_child_link(name, association)
    link_to name, "#", :class => "add_child", :"data-association" => association
  end

  def remove_child_link(name, form_builder)
    form_builder.hidden_field(:_destroy) + link_to(name, "#", :class => "remove_child")
  end
end

default.html.haml

!!! XML
!!! Strict
%html
  %head
    = javascript_include_tag "http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"
    = javascript_include_tag "application"
  %body
    = yield
    #javascript_templates{:style => "display: none"}
      = yield :javascript_templates