In my experience the secret of good backing vocals is good vocal technique. In other words
when singing backing the technique is different to when signing lead. Not only should the
singers not project in the same way, but articulation needs to be altered so that
consonants are less hard, the ends of words are more 'rounded' and the whole sound is
smoother.
Do an internet search for 'backing vocal technique' and you should
find some good stuff.