Just read today in Sound on Sound mag. an interview with the engineer Bruce Swedin and two things struck. MJ would: (i) completely memorize the song lyrics before singing in the recording session and (ii) he would have his vocal coach on hand for a one-hour warm-up session right before. BTW, one of his coaches was Seth Riggs, who has a book out with two CDs – good, but kind of challenging stuff http://www.sethriggs.com/.