I know this one may seem obvious, but I keep forgetting about it.
Look at the beginning of the song and find the first time that the chorus needs to sing the Tonic Chord (the chord that contains doh, mi and soh in whatever key the piece is in.)
Have the people whose notes are doh and soh sing them (with vowels matched) until the 5th locks into place.
Place in the 3rd lightly, then adjust if need be until the chord rings.
Next I ask the chorus to sing this chord over and over until they can get the lock and ring instantly – without having to shift around and adjust for a second or two.
Once everyone has a clear impression of how this feels and sounds, I point out other Tonic Chords a little further on in the song.
They may not be singing the same chord note the next time – but because everyone now knows the feeling of this chord when it’s locked in, each version of it will be in tune. (Though each version may also need to be drilled in the same way as the first)
By drilling this ringing chord again and again, we’re teaching the body to remember how to sing it, in whatever arrangement the notes are written for us. When the body has this figured out, the mind is then free to be more emotionally involved in the song.
And the body loves working on Lock and Ring. It sounds great – but it feels amazing! Guess it becomes like an addiction. The more ringing chords you experience, the more you crave them.
An addiction might explain why Barbershop choruses will happily stand for a 2 ½ hour rehearsal – and with everything memorized, work really, really hard on this stuff.