Template files for specific view modes

Add this to your theme template.php file.

 * adds templates for teasers
 * @param string $vars
function MYTHEME_preprocess_node(&$vars) {
  if ($vars['view_mode'] == 'teaser') {
    $vars['theme_hook_suggestions'][] = 'node__' . $vars['node']->type . '__teaser';

Now you can add a regular template file like node--basic-page--teaser.tpl.php to the templates of your theme. Just make sure the fields you want to use in your template are available in the view mode of the content type.