If singing with pre-recorded music, for monitoring, feed the phones mix into a small FM transmitter.  There are many available for less than $200.  Then have the choir wear portable radios and headphones.  You'll have a few folks who accidentally change the station but it sure saves a bundle on monitoring equipment.  Use radios that lock on frequency if possible.  Also, mark measures in the track in advance. 

Mark obvious starting points in the track in advance.  Obvious starting points would be a few measures prior to verses, choruses and bridges.

On that same note, use the best Mic's, Mic Preamps and A/D converters you can get your hands on.  You'll only get one shot at getting it right.

If space, style and available tracks permit, mic each section separately in addition to your main stereo pair above the director.  The main stereo pair will usually do 90% of the work, but it's nice to have options after everyone goes home.  Spaced omni ambience mics are also advised.

If you are tracking at a venue away from your studio, bring plenty of DI boxes.  Then toss in a few extras.

Be ready to play notes on a keyboard or piano.  Good directors will always want to lead the choir in warmups.  If the group is singing with a track, they may not bring their own pianist along.

Put printed sheet music on stands and try to have copies to the music that will not require page turns.

Cover metal music stands with thick cloth.  Metal music stands ring if not covered.

Pull the music stands up to a high level so the singers don't have to look down while singing.

Take digital photos of the setup and mark locations of singers and mics with tape.  The goal is to be able to recreate the setup later if needed.

Record a click-track to go with the accompaniment.  You might not need to use it but it will often save the day if the choir is having trouble staying together.

If recording with more than a stereo pair, get in the middle of the recording area and click some sticks together a few times.  This will make it easier to time-align digital tracks later.

If the budget permits, get separate takes of each choir part, or just of problematic choir parts.  This gives options later during editing.

Have the choir sing the last note of the song without the high notes if the music calls for them.  Record a separate track for super high notes.  You can pitch correct one soprano on a track by herself.

Most choirs struggle most with cutting off together and blending.  Suggest that the choir rehearse cutoffs and blending extensively in advance.

Be sure you will have enough parking to accommodate the group.

Have plenty of bottled water, AT ROOM TEMPERATURE, available for each singer.  Cold water will not benefit the vocal chords.  If the expense of the water is an issue for you, you can ask the choir to bring their own water.

Groups of people need adequate rest room facilities.  Be sure to stock up on plenty of hand soap (from a pump), towels and toilet tissue.  Some germ-a-phobes find shared hand towels repulsive so a roll of paper towels is also a good idea.  We also recommend putting some hand sanitizer lotion like Purel in there too.  And at the risk of being indelicate, burn a lightly scented candle in the restroom.  We suggest Yankee Candle brand candles because the scents are very realistic but not overpowering and they burn evenly.  We burn candles in each room the choir will be in.

Communicate suggested hygiene tips to the choir members in advance.  Since everyone will be in close proximity, leaving the perfume home will help those with allergies.  However, liberal use of the tooth brushes and mouth wash will be appreciated by everyone.

Lots of bodies in a room create lots of heat.  We've installed flex ducting with in-line duct fans (mounted at the opposite end of the ducting) to expel heat from the room.  We've found the combination quiet enough to operate while recording.

Have stuff to do for folks that are not part of the recording process.  We encourage those recording not to bring anyone that isn't vital to the recording process but sometimes additional bodies are unavoidable.  A few Disney DVD's in another room can do the trick.  Just leave the sub woofer off so the movie doesn't end up in the choir tracks.  And no basketball!

If the session is especially important, video tape.  Be prepared to accommodate three cameras if a video will be made of the tracking session.

Prepare packets of info about the studio.  At a minimum give each person one of your business cards so they can contact you for any future recording needs.