Writing notes
These are things I've noticed over six years of doing this, not rules. Where I've changed my mind, I've left the old note in with a line through the reasoning.
Say one thing
A caption that mentions the weather, the company and how you feel about the year is three captions fighting each other. Pick the one you'd say out loud to a friend and drop the others.
The last word does the work
English lands its weight at the end. "On purpose I took the long way" is limp. "Took the long way on purpose" lands. Same words, different ending.
Punctuation changes the tone more than vocabulary
"Fine." and "Fine" are different people. A full stop on a short line reads as flat, deliberate, sometimes cold. No full stop reads as casual, mid-thought. Neither is correct; they're tools.
Beware the words that have been used up
Journey, chapter, era, soft life, main character. They were all useful once. They now signal that the caption came from a list rather than from you. I used "chapter" myself until about 2023 and it makes me wince to read those back.
Specific beats poetic
"Sunset over the water" is a photo description. "Eight minutes late for the last ferry" is a caption. Detail does the emotional work that adjectives keep failing at.
Read it aloud
If you stumble, the reader stumbles. This is the only one of these I'd call a rule.
A bad example of mine. In 2021 I wrote "Chasing light, finding peace" and put it in three collections. It says nothing, it could go under any photograph taken outdoors, and it was the most shared thing on this site for a year. Popular and empty are not opposites.