selected() was big help for setting a default value. The rest I found in this brilliant meta box tutorial: http://code.tutsplus.com/tutorials/how-to-create-custom-wordpress-writemeta-boxes--wp-20336 with examples for text input, checkboxes and dropdown.
/*** callback ***/
function meta_options(){
global $post;
$custom = get_post_custom($post->ID);
if (isset($custom["reference_id"][0])) {
$reference_id = $custom["reference_id"][0];
} else {
$reference_id = '0';
}
?>
<form action="<?php bloginfo('url'); ?>" method="get">
<select name="ref_id" id="ref_id">
<option value="0" <?php selected($reference_id, '0'); ?>>- choose client -</option>
<?php
global $post;
$args = array(
'numberposts' => -1,
'post_type' => 'page',
'post_parent' => 21
);
$posts = get_posts($args);
foreach( $posts as $post ) : setup_postdata($post); ?>
<option value="<?php echo $post->ID; ?>" <?php selected($reference_id, $post->ID); ?>><?php the_title(); ?></option>
<?php endforeach; ?>
</select>
</form>
<?php
}
/*** save_post ***/
function save_reference_id(){
global $post;
if (isset($_POST["ref_id"])) {
update_post_meta($post->ID, "reference_id", $_POST["ref_id"]);
}
}