Sometimes, a manga marked as completed (and then put in our completed list) gets an extra chapter. How are we supposed to organize our lists in this case, and is there a way to reliably find these extra chapters?

No way

A just ruler amongst tyrants

Not sure if this would help you or not, but if you check the release page once every three days, you will be able to see if someone has released a chapter of a series you have already complete.

It's messy, but it works.

Or... you can go to the IRC channel and make a script that alerts you every time a keyword (such as a series name) is used. Of course, MUbot does not announce "old releases", which can't be seen on the release either.

Of course, you could make a list that is for all your completed series but mark the list as a reading list instead. So, when new chapters become available (as long as they have a higher number than what has already been released), a red number such as "(c.190)" will appear by the series name.

and stuff

[color=green]"Officially, this machine doesn't exist, you didn't get it from me,
and I don't know you. Make sure it doesn't leave the building."[/color]

That last suggestion won't work for new chapters that are listed as "Extra", "Omake", or something similar (chapters without numbers)

A just ruler amongst tyrants

I think Toto's suggestion can work if you select the sort by latest release option. That way any title with a new release will move to the top of the list. Then you only have to check the first few titles (or more), depending on how often you check the list.

Or you can try using this on your completed list. Then you'll get an email if a completed series gets a new release. I never tried this, but it should work.

Looking for... shoujo with a canned peach confession. Don’t English me I’m panic.