I'd say just look at how core does it: http://phpxref.ftwr.co.uk/wordpress/wp-includes/formatting.php.source.html#l1840
I took the liberty of putting the code here for ease of copying and pasting.
global $post;
if( empty($post->post_excerpt) ){
$text = apply_filters( 'the_excerpt', get_the_excerpt() );
} else {
$text = $post->post_excerpt;
$text = strip_shortcodes( $text );
$text = apply_filters('the_content', $text);
$text = str_replace(']]>', ']]>', $text);
$text = strip_tags($text);
$excerpt_length = apply_filters('excerpt_length', 55);
$excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
$words = preg_split("/[\n\r\t ]+/", $text, $excerpt_length + 1, PREG_SPLIT_NO_EMPTY);
if ( count($words) > $excerpt_length ) {
array_pop($words);
$text = implode(' ', $words);
$text = $text . $excerpt_more;
} else {
$text = implode(' ', $words);
}
}