This one sounds a bit silly, but it seems to work....
Put you fingers in your ears (if this is too embarrassing, just try ignoring background noise), then sing a few lines of a song that you're really comfortable with, one that you NEVER have any trouble with. This could be anything from a nursery rhyme to Amazing Grace to The Lady is A Tramp! THEN, without thinking about it too much, just drift into singing the song you're going to sing. Chances are, the first song will have provided a reliable yardstick, and you'll go naturally to the right note.
Oh well, it works for me!!!!!