Adding a wp_head hook from an included PHP file

To enqueue styles and scripts properly, use the wp_enqueue_scripts action like so:

function wpa_63708_enqueue_scripts() {
    wp_register_script( 'my-script','/path/to/script' );
    wp_enqueue_script( 'my-script' );
}    

add_action( 'wp_enqueue_scripts', 'wpa_63708_enqueue_scripts' );