function node_form_build_preview($form, &$form_state) {
  $node = node_form_submit_build_node($form, $form_state);
  $form_state['node_preview'] = node_preview($node);
}

/**
   *Output customized node preview on node edit and add forms.
   *
   * @return
   * This just outputs the Full node ignoring the teaser mode.
   */
  function YOURTHEME_node_preview($variables)
  {
      $node = $variables['node'];
      $elements = node_view($node, 'full');
      $full = drupal_render($elements);
      $output = '<div class="preview">';
      $output .= '<h3 class="post-preview" >' . t('Preview of your posting') . '</h3>';
      $output .= $full;
      $output .= "</div>n";
      return $output;
  }