A Pair of Diamonds
I mark a pair of notes using diamond shapes when I want to remember the first note's pitch to help me play the second note accurately.
I wrote a couple of weeks ago about marking measure numbers in my horn parts. Today, I want to return to the theme of marking up music.
I have experimented with marking a particular note to remind me to remember that pitch. Here is a situation where that was useful for me.

I noticed that I had trouble in rehearsal, more than once, hitting that circled D♭ correctly. Part of the problem, I realized, was that I wasn't hearing the note accurately in my mind before playing it. (Also, many horn players regard that particular D♭ slot as a bit narrow and tricky.)
When a note like this is a persistent problem, I often look for something earlier, either in my part or somewhere else in the orchestra, to help me hear it. In this case, the helper is the same note that appears in my part two measures earlier.

I wasn't having any trouble finding the D♭ in measure 102, probably because of the smaller interval from the B♭ in measure 101. If I could remember the D♭ pitch from measure 102 when I get to measure 104, that second D♭ would be much easier to play accurately.
To help with situations like this, I mark the pair of notes in my part using diamond shapes. The mark is easy to make and easily recognizable. I don't use a mark like that for any other purpose. When I see a pair of notes marked with diamonds coming up, I know that I need to remember the first note's pitch so that it is in my mind when I get to the second note. Here's what it looks like on my part. (Sometimes I use the extra highlighting shown here, but it's not really necessary. The diamonds are enough for me.)

What are your favorite and most helpful markings on your parts?