Custom success message after user is registered

an easy way is to redirect form submitter

// when form is submitted :
wp_redirect($your_post_url . '&submitform=1');
exit;

Then you can just test if “submitform” is defined to display what you want :

// test submitform url setting :
if (isset($_GET['submitform'])) 
    echo '<div class="successmessage"><p>' . __('Your Success message.') . '</p></div>';
endif;