By default, when you set up a child menu, it appears directly under and slightly indented from the parent item. Occasionally, there is a need (such as with the kiosk I'm currently working on for the Christmas Island Tourism Association) where you need an active-trail but the menus need to be in different regions.
Rather trying to do everything with blocks in regions I found it easier to dump code in templates.
<?php
$main = menu_navigation_links('main-menu', '0');
echo theme('links__system_main_menu', array(
'links' => $main,
'attributes' => array(
'id' => 'main-menu',
'class' => array('links')
),
));
?>