Okay, so. To put it simply:
I have a character with a fairly pronounced facial twitch. I feel like it would look strange(er) and distracting to have his facial expression change every panel, but I'm not sure how else to write it in a way that would call attention to it but not so much that it is persistantly annoying or distracting.

I'm trying to convey this the same way that the other characters see it-- noticable at first, but eventually something that they get used to-- but I'm just not sure how to write it.

Occasionally draw the twitch in two or three panels for emphasis, especcially the first time it happen, but at other times work it into a single panel.
Ways to do that are, that come to my mind right now are these two:
Draw two states of the face in the same panel, so it is clear that the face quickly changes between those positions, especcially when the twitch was elaborately pointed out earlier in the comic.
Occasionally draw the face in a single position indicating/showing that the twitch is taking place.

Good luck!

Hmmm, difficult. What IS the twitch exactly. If it was an eye-twitch I'd have the character drawn with one eye closed every third panel they were in or something like that. With a mouth-twitch every third panel with his mouth twisted for some reason (in a snarl, half-smile or whatever you think best).

I'd definitely have another character mention it early, so it doesn't look like some strange quirk of the artist.

Twitch's are both involuntary and reactionary. I think the involuntary twitch's you could just leave as a twitch or not even draw them. The reactionary draw attention to it with maybe a couple lines radiating out. But definitely have the twitch there. Lines work good after the first couple panels the readers will associate the two and then you don't need to draw face shot all the time. Just the reaction line.