In proper classical singing, space is always vertical. What does this mean? Singers must always strive to keep the space in their throat, mouth and nasopharynx as tall and narrow as possible; nothing should be spread horizontally.
There are several reasons for this. First, it's more comfortable because it reduces strain on the vocal apparatus; it involves fewer muscles, instantly increasing vocal endurance. This is tremendously important for Orthodox choirs because we're singing almost non-stop throughout the services. Singing while vocally fatigued doesn't just decrease vocal range and produce a tired, lackluster sound; it can inspire singers to push, causing pitch problems and leading to potentially serious vocal injury. It's important to know that the larynx, like the eye, doesn't contain a ton of nerve endings. Therefore, singers who incur a vocal injury often feel no pain, leading to delayed treatment and further vocal damage. Unfortunately, some forms of damage are permanent. Keeping vocal space tall and narrow also assists with choral tuning vis-a-vis vowel unification, and increases the intelligibility of the text, which is of paramount concern in the church.
The most effective and straightforward way to get singers on the "tall and narrow" bandwagon is to ask them to sing with a British accent. British vowels are naturally tall and narrow, but that's not the only perk: singing in a proper British accent also virtually eliminates the problematic American "R, which can create intonation problems — and frankly can result in some ugly singing. In fact, the treatment of "R" is perhaps the most noticeable difference between British and American diction.
When directors first ask the choir to sing with a British inflection, some resistance is likely (I even had one singer walk out of rehearsal over this, and sadly they never returned). The singers may be concerned about sounding pretentious and affected, but a simple demonstration or two gets most people on board. My method is to sing something quick and simple (like a troparion) without changing any of the diction, and then to pencil in the British bits and do it again. THEN I reveal that I'd recorded both on my phone, and play them back for the choir. Point made.
Interestingly, I've found that British diction is toughest to implement in "throwaway" arrangements such as litanies. Let's just admit it: we've all been guilty of going on autopilot on these. The thing is, litany responses are tiny little minefields because of the words "Lord" and "mercy". When we speak those words, the American "R" isn't a problem, but when extended on a musical pitch, we suddenly find ourselves at the Grand Ole Opry. (St. Dolly Parton, pray for us.)
One final word about pretension. It's important that singers understand that while singing with a British accent may feel very alien to them, it doesn't sound that way to the listener. There's obviously a change in the sound — that's why we're doing this, after all — but rather than sounding snobby or affected, the choir simply gains greater cohesion, better tuning and a more professional overall sound. Repeated recording and playing back will help reassure the singers that they don't sound ridiculous. :)