The most important part of choir directing is the teaching. The congregation sees the director at the end of the process, when the choir sings, but the real work of the director is what happens in rehearsal. Everything the choir sings comes from what they've been taught. When you come before the choir to teach, you want them to feel confident that you're going to share something good and worthwhile with them.
What this means for you is that you need to be fully prepared to teach. The rehearsal is the choir's time for learning and practice. But the director's time for learning and practice is BEFORE the rehearsal.
Spend time alone with the music, getting familiar with the songs you're going to teach. You want to know the words by memory so that you can make eye contact with the choir while you teach. You want to know all of the vocal parts so that you can teach them with confidence.
What's the best way to learn the words and the parts? Practice singing the song! Sing it out loud, over and over and over. Sing the soprano line; sing the alto line; sing the tenor line and the bass line. Singing the song yourself helps you memorize everything. It also helps you recognize which parts of the song are the most challenging so you'll know what parts you might need to spend more time on during rehearsal.
Practice the song over the course of several days. You might work on it one day and feel like you completely know it, but then a couple days later there will be parts that you've forgotten. Your brain does much better at keeping information in long-term memory if you practice at several different times.
When a director is well prepared, the choir members know that the rehearsal is going to be productive and worth their time. If it seems like the teacher is unsure, it can be very discouraging to the choir. No choir member wants to see the director playing the CD during rehearsal trying to figure out how a line is supposed to go!
Now, in some choirs, the choir director is not the one who selects the music. If you have a music director who chooses the songs for the choir to learn, communicate with them about how much advance time you need to prepare songs before you teach them. I work with one choir where a music director picks the music for me. I try to prompt him well in advance about important occasions: “What songs do you have in mind for Easter this year?”; “Did you want to do any special music for Mother's Day?” The sooner I know what we're doing, the better I can be prepared, especially for any difficult music.
The Bible says that studying will make us into a worker that doesn't need to be ashamed. This principle definitely applies to the music ministry. The more we study, the less we'll need to apologize for when we stand up to teach the choir.