(Submitted by Michael Mauldin, September, 1999, as a post in response to a question by another member of the forum):
You are right when you say that one can quickly get a feel for a choir's own distinctive sound through hearing its unison. For me, vowel placement is the key.
I use a what some call an "oo-centered" approach. I often start rehearsal, after the initial stretching, breathing and warm-ups, with asking the choir to sing a G or A above middle C (in a comfortable "buzz" zone for the head/mouth-voice mix for both soprano and alto). I ask them to relax and to breathe when needed, but to keep the vowel going constantly (this can take four or five minutes). We start with an "oo" and stay with it until it "feels" right (until everyone has roughly the same amount of openness of the jaw and lips, and of course, is on the same pitch and using support). I will tell them that we are about to go to "ah" and that when we do, they are not to move much of anything but their lips. The mouth should already be almost open enough to sing "ah" effectively. When I give the signal to go to "ah" many of them open too suddenly ("letting the dam break") and we return immediately to "oo". We prepare again, and see if we can smoothly get all at once to a good, open, relaxed "ah" with that "feel"--I wait until they all hear and FEEL the buzz of having matched vowel placement. Then we return to "oo", then proceeding to another vowel until its transition from "oo" is smooth and uniform, then back to "oo" and on to another vowel. There are always differences of opinion on vocal technique, but this seems to work to help unify the sound of the unison.
--Michael Mauldin, Albuquerque Boy Choir