I've been trying to think of how my church does it, because it works out perfectly. I don't know that there really are any rules to it; it just happens.
We have a potluck every month there, so it's a pretty large group. The city is already kinda divided into sections, so each section has a role - the north area will be providing the main course, for instance, while the south area will be providing the desserts. That type of thing. So everything is covered with the meal. The duties of setting up and taking down rotate as well - I think the south part did it last month. So that's divided up fairly.
It's right after the later service, so people are kinda milling around getting their contributions on the tables and getting things ready. People with mobility issues or elderly people are where they can get to the tables easily anyway.
There are 2 tables set up, and people go on both sides of the tables. So the line moves incredibly fast. Now, that means that you might not be in the line where you get Martha's meatloaf on the other table. But you are getting Sue's homemade soup. And, if you want some of the meatloaf, you can either slip in and get some (people don't mind a quick "hey, could I grab a quick bit of this?" type of thing) or go back and get some when you get seconds.
I have never seen the food all disappear at those things. And no one is leaving the slightest bit hungry - I have left there stuffed to the gills!
These people make enough food to donate - which is exactly what's done with the leftovers.
I think there was one time where they did kinda have people seated at tables (if I remember correctly, there was an announcement or something that took place before it, so people were already sitting) and then they did a kind of start at this table and work around the room to get in line. But, even then, it wasn't a strict thing.
The thing I've noticed about how it works is that there isn't a problem, like Snowdragon mentioned, of Singles being left out. They're in the line just like everyone else. It's a bit of "first come, first served," but it doesn't make the ones who may need a little extra time be in the very back or be at a disadvantage (as Slitherhiss mentioned).
Somehow, it all works! This church has found the magic touch that makes it go really smoothly. Part of that might be the fact that it is a monthly thing. There's been enough practice now that they've worked out the kinks, and people know what to do.
ETA a missing "doesn't" that was needed