Darn, I can't remember where I stole this from. Either @bgardner or from my.studiopress.com, I think. Anyway, It's really straight forward but I use it all the time. I like to avoid mixing my style changes / additions to the theme as provided.
// Load custom style sheet
add_action( 'wp_enqueue_scripts', 'custom_load_custom_style_sheet' );
function custom_load_custom_style_sheet() {
wp_enqueue_style( 'custom-stylesheet', CHILD_URL . '/custom.css', array(), PARENT_THEME_VERSION );
}