In my experience, it's best just to stop right when you notice the issue and address it immediately.
If you are determined to continue without stopping, and your quick slash isn't enough to jog your memory, either stop conducting, letting your musicians continue, and actually write a note, or just don't worry about it.
If you don't notice anything the next time through, your musicians fixed it on their own, and if you do notice it again, it will be easier to remember.
music.stackexchange.com/users/65/gilly3
I have no experience in conducting, but I assume what works for your own instrumental performance will work here, too.
When you're really in doubt, I would definitely try to record the performance and listen to it again when you can totally concentrate on it.
You'll have more "space" to analyze and the next time you'll be more prepared.
music.stackexchange.com/users/70/Scorchio
I would say that the best way is to try to go through the whole piece at least once, marking each issue to address with a small but noticeable sign in the conductor's score (I would recommend a circle), so that later you can study each of them by its own, and with the proper musicians.
Be careful, though.
Don't fall into the trap of trying to correct every little detail the first time and
also you'll see that sometimes the same musicians will notice that something was off, and they might try to fix it themselves.
Also, at the beginning of the following rehearsals, go directly to one of the issues you marked last time (preferably the one with the most difficulty)
music.stackexchange.com/users/101/Edgar Gonzalez
Keep a small personal recorder on you, with a one-button start-stop interface, and an external mic attached to your shirt collar.
If something happens, press the button and mention something that will jog your memory when you review.
If you have a smartphone, there's most likely an app in the app store that will serve this purpose well for you
, you can use the button on the phone's in-ear headset to control the recording, or a large button displayed prominently on the screen that will be easy for you to press.
Alternatively: if you can find someone who'd be interested, get an assistant conductor who can stand near you and take notes.
Then you can tell the assistant a few keywords, and assume that they're following along on a part and can jot notes down in the correct place for you.
This solution will make it easier to review the notes with the ensemble right away.
music.stackexchange.com/users/6/Jason LeBrun
Our choir director has a "three strikes and we're out" rule, because she can remember two problem spots in a song before stopping at the third.
I'm learning to conduct from her and one trick I've picked up is to stick my finger on the page where a problem happened; I can track one that way.
Usually the reminder of where it was is enough, since most of our songs are short (3min or under).
For longer songs I would stop at the first problem that I want to address now, which might not be the first problem to occur.
music.stackexchange.com/users/480/Monica Cellio
A small idea from one who doesn't conduct, but records a lot.
I've sat through plenty recordings checking the scores whilst recording.
Always make the note on that section/instrument that has the problem.
Write only on the top of the page, if it's a problem for the whole orchestra.
Make your own system of terminology.
A circle can mean dynamic problem, a box can mean rhythmic problem, triangle is something something.
Assuming your score is on off-white paper printed with black (possibly laser) use a green or red (if you're colorblind, they are still the same) marker or pencil, so it sticks in the eye on page turn.
I use a pen cause
once recorded I don't really need the score anymore, but if you need it for the next day, you might want to go home and erase the points.
One note about recording: it doesn't help to start recording when you notice something.
Record the WHOLE rehearsal and listen to it when you get home.
Looking at the score and seeing your points a couple of hours later, will give you a good idea of what you heard and what they actually played.
music.stackexchange.com/users/2241/Thomas Bryla
I've seen conductors mark trouble spots with Post-It flags, but haven't tried this myself.
Yet. :-)
music.stackexchange.com/users/2229/SarekOfVulcan
As a mostly untrained conductor, one problem I've always had is keeping track of problems with a piece as we work through it.  Something will stick out  and I'll want to go back, but all I could manage was a slash or other simple mark that isn't enough information to fully recall the issue.  Is there a common method for recording musical issues for later correction?
Keeping track of problems during rehearsals
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music.stackexchange.com/questions/25
music.stackexchange.com/users/41/derekerdmann
S0: I would say that the best way is to try to go through the whole piece at least once, marking each issue to address with a small but noticeable sign in the conductor's score (I would recommend a circle), so that later you can study each of them by its own, and with the proper musicians.  Always make the note on that section/instrument that has the problem.  For longer songs, stop at the first problem you want to address at the time, which might not be the first problem to occur. When you're really in doubt, I would definitely try to record the performance and listen to it again when you can totally concentrate on it. Keep a small personal recorder on you, with a one-button start-stop interface, and an external mic attached to your shirt collar. Alternatively, if you can find someone who'd be interested, get an assistant conductor who can stand near you and take notes. To note a musical issue, you could either record the performace and use an external microphone to add comments, or you could write notes on the scores, using symbols/notes/post it notes,  possible stoping conducting breifly, or use an assistant to take notes.
To note a musical issue, you could stop conduction, write a note, use signs, your finger, or post it notes on the relevant position Stop the performance at the issue you want to address Record the whole performance, use an external mic and listen after  Alternatively: if you can find someone who'd be interested, get an assistant conductor who can stand near you and take notes.