I'm trying to find symbols and tools and basically anything visually iconic of hypnosis, trance, and MC.
I'd like a little help if you can spare the time. If you think you have something obvious or even vague post it anyway. I'd like to make a sort of comprehensive list.

In what context you're talking about is left a little vague, but...

Well, you have the really common stuff such as:

* pocket watches or other pendulums.

* spirals in general. Doubly so if they're actively spinning.

* odd eye effects - glowing, empty, spiral...

* that expression I like to call "dumb face". You know the one. Mindlessness.

Then you have the slightly more abstract stuff such as:

* anything that could be iconic to the presence of "subliminal messages".
** Some people find an MP3 player and some earbuds enough to symbolize such. Too thin a symbolization for me, though.

* really weird electrode and LED light covered helmets with tons of wires. Because mind control helmets.

* really weird and tech-looking collars with tons of lights and a monitor screen or something, because mind control collars.

* any accessory sharing an odd glow with eyes, because enchanted mind control accessories.

There's more, I'm sure, but I don't wanna break the thread by bankrupting the ideas. :P

Going off of what Vanndril mentioned about mind control accessories, puppet strings or ritual markings/tattoos can work. The last one can also lead to delicious corruption~

For more comical effects, I was thinking of a wind-up key getting stuck into someone's back and making them act like a wind-up toy or doll. I know I've seen that on Looney Tunes or something.

Ah, and now that I think about it: Kaa. Although Kaa isn't my thing. :P

Pocket watches, spirals, catalepsy (the image of a person stretched across two chairs, for instance), Kaa eyes, snakes in general, the whole "balloon tied to your arm" pose, collars, clucking like a chicken, empty eyes, zombie walking, the phrase "yes master"... I'm sure I can think of more.

Everything can be a hypnotic accessory if you're brave enough.

rarely, you see change of skin tone and change of skin texture

For some reason, the color green really seemed to resonate with people for awhile. In general you see a lot less hot colors at work.

Goo, injections through skin/ears/cranium, any glowing fluid that touches the subject's body.

TENTACLES! (Associated with any hypnotized expression)

Tentacles with an hypnotic eyeball at the tip.

I think that puppetry imagery is the most classic way to show that someone (or usually, many individuals) are being controlled. Vacancy in the eyes or just them looking weird is also super iconic.

Ugh, hypno-planking. I remember when that crap took over the DA_Entranced group for a while, the dark age of cheap-o "stiff as a board" manips.