application.js

$('.action.new[data-content][data-association]').click(function () {
  var new_id = new Date().getTime();
  var regexp = new RegExp("new_" + $(this).attr('data-association'), "g");

  $(this).before($(this).attr('data-content').replace(regexp, new_id));

  return false;
});

application_helper.rb

def new_object_fields(f, association)
  new_object = f.object.class.reflect_on_association(association).klass.new
  f.fields_for(association, new_object, :child_index => "new_#{association.to_s.singularize}") do |builder|
    render(:partial => "#{association.to_s.singularize}_fields", :locals => {:f => builder})
  end
end

complex_form.html.haml

= form_for @parent do |f|
  %h3 Children
  %fieldset.children
    = fields_for :children do |children_form|
      = render :partial => 'parent/children_fields', :locals => {:f => children_form}
    = link_to 'Add Child', '#', :class => 'action new', 'data-association' => 'child', 'data-content' => new_object_fields(f, :children)