I don't have a choir at the church where I mix all the time. But when I do there's a few things that I always bring up as a reminder the first couple rehearsals that they're in for. There's the usual stuff. Get in a nice group on the risers, don't bunch up or spread out too much, make it look intentional. If the monitors don't sound right, speak up or I won't know to change them for you. And one last thing.
Take it easy with the clapping. I use some pretty sensitive mics and tune them up and process them to the max to get the most out of the choir vocals. They're competing against a seven piece rock band and a handful of other vocalists with their own mics. Those puppies are hot! (The mics, not the singers, we have really good HVAC in our room.) So when they start clapping enthusiastically it pretty much sounds like gunfire out in the house.
So we always do a quick lesson on stage clapping. You don't have to do a total soft clap, some noise is alright and fits right in the mix. You can clap one palm on the other wrist or clap both wrists together. That way the choir still looks enthusiastic and the audience can see that and hopefully be moved and inspired.
It usually works great. Only once in a great while does somebody forget and just wail away with abandon. And that's just a quick lesson that there are some things that just can't be fixed in the mix and your engineering becomes social engineering. Just keep in mind that when you need to get musicians to do something it needs to appear that you're on their side and not just trying to manipulate them.