Here’s a simple vocalize designed to help your choir concentrate on proper breathing.
Have them breathe in using their diaphragm and let out the air on an “s”. Exhaling on “s” slows down the rate of expiration enable your choir to experience just how long they can hold a note, and it gives the singers a chance to really feel the diaphragm at work. Also, exhaling on “s” doesn’t strain the vocal chords in the way singing on a pitch does. After some amount of time (say 5 seconds) have them exhale the rest out.
Check to make sure that the singers aren’t raising their shoulders while inhaling. Doing so is an indication that their diaphragm isn’t really at work. Another thing to look out for is tension in the abdomen muscles while exhaling. This tends to happen when singers are trying to create the sensation of “breath support”, and ends up increasing pressure on the vocal chords. Not a good thing.
I found this technique in a few books and have had a director use it as a warm-up exercise. I like the exercise a lot because I can really concentrate on one thing, proper breathing, without having to also think about things like pitch or dynamics. In my opinion it’s good to have some exercises like this which isolate a certain aspect of singing for study and some exercises that challenge the singer to execute several complicated tasks at once. The “Now Wolde I Fayne” exercises fall into the latter category.