application.js

function remove_fields(link) {
  $(link).prev("input[type=hidden]").val("1");
  $(link).closest('.repeating_fields').slideUp();
}

function add_fields(link, association, content) {
  var newId = new Date().getTime();
  var regexp = new RegExp("new_" + association, "g");
  var newItem = $(content.replace(regexp, newId)).hide();
  $(link).prev('.fields').append(newItem);
  newItem.slideDown().find('input:first').focus();
}

edit.haml

- form_for @mortgage_application do |f|

  %fieldset#mortgage_fields
    %legend Existing Mortgages
    %ul
      - f.fields_for :properties do |fa4|
        %li.repeating_fields
          %fieldset
            %legend Mortgage Details
            = f.label :lender
            = f.text_field :lender
            = f.label :mortgage_number
            = f.text_field :mortgage_number
            = f.label :agent_name
            = f.text_field :agent_name
            = link_to_remove_fields("Remove", f)
    = link_to_add_fields "Add another mortgage", f, :properties

helper.rb

  def link_to_remove_fields(name, f)
    f.hidden_field(:_destroy) + link_to_function(name, "remove_fields(this)", :class => "icon_button only_icon_button remove_button")
  end

  def link_to_add_fields(name, f, association, options={})
    options.merge! :class => "icon_button text_icon_button add_button"
    association_class = f.object.class.reflect_on_association(association).klass
    new_object = association_class.new
    fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
      render new_object.class.table_name.to_s + '/fields', :f => builder
    end
    link_to_function name, h("add_fields(this, '#{association}', '#{escape_javascript(fields)}')"), options
  end