The same images recurring too often also happened to me for programming as well as math.
The imagery would start to bleed into each other and I’d forget which particular variation I was attempting to recall.
With this in mind my strategy is to find themes, and to pull from a set of images with that theme.
For instance, I use both ‘crab’ and ‘Krusty the Clown’ for the number 74. That’s an example of a number that has a ‘set’ of images.
For the plus sign, I can use the cross (naturally) but I also sometimes turn it into a stake (for stabbing vampires and such).
This keeps the imagery still resembling a cross and matching the cross-theme.
For parentheses (especially in programming) I use a cauldron.
(see how much stuff fits in this cauldron?)
But I’ve also used a hula hoop
(ab)
^look how they’re stuck together in that hoop
I also have used a lasso. The theme is “things grouped together in there” I suppose.
For division I have used a samurai “dividing” with his blade /
But a pole launched through the dividend by the divisor works too. As long as the dividend ends up ‘“divided”.
For the term equation, I use a horse (equine).
But equality uses a koala.
If equation comes up a lot, terminology wise, I keep with the horse theme somehow.
There’s a couple examples of how I think determine which mnemonics to use for commonly repeated terminology and symbols. I guess you could say I don’t pick a single image so much as I play off of motifs.