I am hacking together a theme for wordpress and I am using the following code to pull out data from a custom field with several values:

            <?php  $mykey_values = get_post_custom_values('services');
            foreach ( $mykey_values as $key => $value ) {
            echo "<span>$value, </span>";
            } ?>

I use a comma to seperate the results, but I don't want a comma after the last result. How do I get around this?