Include print style sheet

Use wp_enqueue_style to add a print stylesheet, note the media parameter that lets you make it a print-specific stylesheet:

function wpa_print_styles(){
    wp_enqueue_style(
        'wpa-print-style', 
        get_stylesheet_directory_uri() . '/print-style.css', 
        array(), 
        '20130821', 
        'print' // print styles only
    );
}
add_action( 'wp_enqueue_scripts', 'wpa_print_styles' );