Here is a non-comprehensive list of practices I have found to make choir rehearsals more effective and efficient. I am not an efficiency expert by any means - the rehearsals at which I preside often side-track, but that's just my personal directing style. However, when the choir needs to buckle down and do some Hard Work, I have some cards up my sleeve.
Find the right time of day. Schedules are a choir director's biggest challenge. If you are directing a parish choir of mostly adults or older students, you're almost certainly going to land on a weekday evening. Be sensitive to your singers' mental cutoff point - ours is 8:45. If your choir members stop processing after 9pm, you won't accomplish anything by pushing on to Just One More Time. For those big pre-Major-Feast rehearsals, make it a morning; twice a year I bribe my choir with coffee and donuts and sing for three hours on a Saturday morning, once before Christmas and once before Holy Week. Those are always our best rehearsals, and if people didn't have things like sports and household chores and families to spend time with, we'd rehearse Saturday morning every week.
Begin and end with prayer. You can use the prayers for rehearsal that were the first thing I posted on this blog (because it is the most valuable), or something else. A definitive start and end to practice maintains a sense of tidiness and completion, and of course prayer is a primary element of singing in choir in the first place, a reminder of Why We Are Here.
Have way more copies than you think you need.* People forget, lose, deface, and improperly file their music all the time. Breaking rehearsal to run to the copier costs time and momentum, and singers who crane their necks to share with their neighbours lose tone quality through the strain. Have it all ready and laid out, preferably near the entrance to your rehearsal room. And invest in a good quality 3-hole punch! (There are those choir directors who will prepare everyone's folders for them before rehearsal, and collect them at the end, but this requires way more organizational capacity than I possess, in addition to depriving choir members of the benefit of taking their own notes. More useful for a children's choir, probably.)
Start with physical and vocal warm-up. At bare minimum, the choir should stand, stretch, and whoop - starting at the extreme bottom of their ranges, just toss your voice all the way up to the top a couple of times, trying to get a bit higher each time. Then sing your easiest hymn first, standing up. Arm, neck and torso stretches are also good. Tight muscles mean tight voices. Singers who pay attention to the way their bodies feel during singing will have more stamina and better tone.
Use a piano (if you have one). The pipe organ is a perfect instrument for sustaining congregational singing. For much the same reasons, it isn't very good for teaching singers their parts. The piano, by contrasting with the sound of the voice, can be picked out and followed. The percussive nature of the tone makes the rhythmic patterns obvious. It doesn't have to be a good piano, but it should be able to be tuned enough that it isn't painful to listen to. (If you do have to use an organ, use a bright setting for picking out the parts - a couple of 8s and a 2 - but no stops labeled "celeste" or "doux," as these are tuned slightly off-pitch to achieve certain effects. Avoid mixtures or stops with fractions for similar reasons.)
Group similar things together. This applies to musical styles as well as to voices. Sing all your hymns, then move to your chants, then work on polyphony. Even (or especially) if you are practicing for multiple liturgies, as you might do for a Holy Week rehearsal (or if a Solemnity falls on your practice night so you're losing one and no one has time for an off-day rehearsal), keep to this order. Shifting styles requires mental energy, and you want to save that up for learning the notes. Similarly with voices, when you are learning new lines, rehearse each line separately, and then in pairs. With hymns, the most intuitive pairings are usually S-A and T-B, but with polyphonic pieces, I've found that S-T and A-B works better most of the time. Oftentimes the closer parts will cross each other in a confusing way, and both parts are easier to hear when they are in different registers. Also those pairings just have a better affinity. I seat my choir this way too - altos in front of basses and sopranos in front of tenors.
Keep singers busy. Switch between sections often, breaking up your new motet into chunks so that everyone gets a chance to sing at least every 5 minutes. Idle singers are chatty singers, or worse, bored singers. (As a side note, I have actively encouraged the passing of notes among the younger set, to cut down on non-singing noise. They should be having a good social time, but hey, we need to work here, people.) Sometimes singers who are not "on duty" will hum their own lines quietly along with the section currently learning. You can decide when this is distracting and when it is not, and you don't have any obligation to be consistent with this. Meet the needs of the moment.
End with something that highlights your accomplishment. Singers should leave rehearsal feeling like they accomplished something. If your Sunday prep doesn't take up your whole rehearsal, work on something for the future. If it's something tougher than you usually do, find something that breaks up into small bits so that you can really learn one section at a time - something like this, which was the first major polyphonic work I taught to my choir. Each 4-7 bar chunk can be worked up successfully by itself in a single rehearsal, then strung together to boost your choir's confidence. Alternatively, find a short, uplifting hymn that everyone knows and use it as a rehearsal closer.
*Even if you use books instead of handouts, have more spare books on hand than you think you need, both in your practice space and in the church. Remember to ask your choir members to leave behind the ones they borrow if they have their own at home.