Minimize Verbal Instruction
A rehearsal technique that has popped up at several conducting workshops is the no-talking rehearsal. The idea is to minimize the amount of talking that the conductor does and communicate non-verbally as much as possible.
Here's a fantastic example in a Harmony University video featuring Don Campbell. I've watched this video at least a dozen times. It's kind of amazing how he can drive the rehearsal through conducting gesture alone. Occasionally I try to emulate this rehearsal technique, although I tend to use more singing demonstrations.
When I employ this technique, this is what I observe:
The singers get to sing more (almost always a good thing)
The singers are more attentive. You can't look away, you can't be buried in the sheet music, you can't check your phone... If you do, you'll miss the visual instructions.
I get to rest my speaking voice (at the end of the night, my voice is less tired, which is nice)
A singing demonstration can often do the work of a verbal instruction, but quicker.
Also, a singing demonstration is information-rich: it can correct pitch errors, model good vowels and tone, correct rhythms...
A non-verbal signal (or 'emblem') can be used to remind the singers of specific concepts. For example, in this video I use an arched hand position for the 'EE' vowel sound (stole this directly from Don Campbell - he has a 'vowel continuum' which is a very useful rehearsal tool).
I use the opportunity to use sol fa/solfege hand signs for the singers to follow. Even if the singers aren't familiar with the system, they pick up the meanings by observing.
One thing to note - it's much faster if you control the pitch source in this rehearsal scenario. In my case, I use a little keyboard on a desk in front of me. Other times I've used a pitch pipe. But it's always controlled by me (as opposed to a singer in the choir, or an accompanist).
Some pre-requisites include: knowing the music very well; knowing how you want it to sound, having a reasonably clear and effective conducting gesture, being able to sing and demonstrate the sound you want for each part.