A good rehearsal starts with everyone knowing what's expected in advance. They need to know what version of the song you are doing. I suggest the studio version as its the one people normally hear. Make sure as BL you know that structure cold. Make sure it's communicated in advance that everyone should come knowing their part. I like to write it out for the group in whatever format they can read. Jazz musicians -- a lead sheet, certain rock musicians, sometimes lyrics with chords over it, or a chord sheet. Some can't read a chord sheet, so having it as lyrics and chords over top works. At times I've given them the Ultimate Guitar Tab lyrics and chords and we follow that. I sent that out in advance, depending on the skill level of the players. With some, I can just give it to them at rehearsal and they nail it cold; others, I have to give it to them in advance.
Resist like the plague attempts by the band members to change the structure from the studio version unless there's a really good reason. I've been in rehearsals where the song structure is negotiated. Communication is H-E-double hockey sticks because one person means the bridge and someone else means an instrumental break. The band rehearses the song 6 times, exposing all the communication problems, and then somehow everyone gets the structure right. Everyone declares victory -- and then promptly forgets the structure for next rehearsal. Don't waste time on that! Have the song structure established and good reasons for doing it that way:
1. People can learn independently from the Youtube video.
2. It allows you to create a playlist to sub new players when people aren't available
3. It makes rehearsals shorter and more productive so you can get out and gig faster.
4. It retains members when they have a good practice.
If you have a fledgling member and you need to mentor/train them, get them there early to go over what you need to show/teach them. Don't waste other members time in group rehearsal. Group rehearsal is for Group rehearsal -- group issues like time, dynamics, getting tight etcetera. In a pro band this is unnecessary and never even customary, but I have groomed subs and new players to increae musician supply in the past.
In rehearsal, listen carefully. You should know your part cold. Then listen for places the group isn't together, for bad notes, rushing temp, dynamic issues (places you should get quiet and soft or loud and energetic to create drama), accents or other things the band missed. After a while you'll be able to forecast what they will have trouble with and have clear guidance in your head to share for how to fix it before you even get into rehearsal.
Another thing -- try what is called interleaving. I can get my band further along the path to competence when we do our rehearsal like this
Song1
Song 2
Song 3
Song1
Song 2
Song3
Song1
Song2
Song3
It's the retrieval of the information that makes for permanent learning and good habits. Therefore,you want the band to retrieve the song as many times as possible in rehearsal -- after having a chance to forget it. A less effective way is the way most bands do it...
Song 1 a buzillion times
Song 2 a buzillion times
Song 3 a buzillion times
This method is not nearly as effective as the interleaving method I shared above where you rotate through songs you don't know well so musicians have to think and retrieve.
Make sure there's time for fun and giggles, jokes, sharing how things are going.
If someone shows up unprepared, use your interpersonal skills to show disapproval while still being friendly and kind. Be careful, as they are probably volunteers, or close to it, but make sure you show some kind of disapproval like "Ouch, that means we won't get as far as I thought today" and wince. Or something kind, but indicative that showing up unprepared is unacceptable. Use your personality but don't be a dick about it.
In extreme cases when everyone confesses they didn't learn the songs beforehand, I have on occasion indicated I was well prepared, but don't see the point in rehearsing if no one else or the majority is prepared. Suggest everyone goes home and uses the time to learn the songs. Reschedule.
That sends a really clear message that showing up for rehearsal isn't worth the drive over there!And that Mr. BL expects you not to waste everyone's time. I wouldn't do this in first rehearsal -- give people a chance to learn the norm of preparedness through your body language at first. You don't want to kill the goose the first time it tries to lay an egg.