Manually highlight Wordpress admin menu item

To further clarify Ken Vu's answer, edit the global variables $submenu_file and $parent_file. E.g., to highlight your page:

global $submenu_file;
$submenu_file = "edit-pages.php?special-pages=true";

If you need to change the top-level item highlighted, use $parent_file. E.g., highlight the "Writing" setting page:

global $parent_file;
global $submenu_file;
$parent_file = 'options-general.php';
$submenu_file = 'options-writing.php';

better solution:

add_filter('parent_file', 'my_plugin_select_submenu');
function my_plugin_select_submenu($file) {
        global $plugin_page;
        if ('__my-current-submenu-slug__' == $plugin_page) {
            $plugin_page = '__my-submenu-slug-to-select__';
        }
        return $file;
}