Styling active links submenu

Hello,

I’m trying to modify the standard theme a bit. I want one of my links to display a custom made sub menu. Here is the code that generates the sub-menu:

<?php //submenu
global $wp_query;

if( empty($wp_query->post->post_parent) ) {
$parent = $wp_query->post->ID;
} else {
$parent = $wp_query->post->post_parent;

}

?>

<?php if(wp_list_pages("title_li=&child_of=$parent&echo=0" )): ?>
<div id="submenu">
<ul>
<?php wp_list_pages("title_li=&child_of=$parent" ); ?>
</ul>
</div>
<?php endif; ?>

It works, but is this the best way to make a sub-menu? And how do I keep the main links active when a sub menu item is clicked?

And my final question: How to keep the right sub menu item active as well?

Thanks