public function saveAction()
    {
        // get the $showTextField from the POST if there is one
        if ($this->_request->isPost()) {
            $showTextField = $this->_request->getPost('show_text_field');
        } else {
            $showTextField = 0;
        }

        // get the form object
        $this->view->form = $this->getForm($showTextField);
        
        // rest of the controller logic goes here
    }

    private function getForm($showTextField)
    {
        // ...create the form object here

        // show_text_field
        $options = array(1 => 'Yes', 0 => 'No');
        $form->addElement('select', 'show_text_field');
        $form->show_text_field->setLabel('Show Text Field:')
                              ->setOptions(array('onChange' => 'submit();'))
                              ->setRequired(true)
                              ->setMultiOptions($options)
                              ->addValidator('Int')
                              ->addFilter('StripTags');

        if ($showTextField) {
            // first_name
            $form->addElement('text', 'first_name');
            $form->first_name->setLabel('First Name:')
                             ->setOptions(array('size' => 30,
                                                'maxlength' => 50))
                             ->addValidator('stringLength', true, array(1, 50))
                             ->setRequired(false)
                             ->addFilter('StripTags');
        }

        return $form;
    }