Since most of book covers on research mathematics is intentionally kept sober, to make a poster you may want to keep it simple yet not too boring. Also, it depends on the subject, audience, medium and budget.
keep it simple or to borrow the term: minimalistic
Visual wordplay or pun (as long as it is not overdone) OR, allusion to artwork such as Magritte or Escher if you are dealing with a topic that is self-referential, but avoid being cliche. Since mathematics is hotbed for symbols and logos, you can alter the font to mold into an object. The zero-sharp, zero-dagger, club suit, diamond suit, etc. Example, a bird morphing from "w" (the smallercase Greek omega letter)
You can also use an equation in a Rebus style such as the much abused meme of $i$, complex number and irrationality. Although memes can be rather cheap humor, you can browse to keep ideas flowing. This cartoon was quite interesting. (It can also be a simple Euler's identity with the tagline: Thus God exists.) Also, try to make a campaign - to borrow advertising term- so that you keep one constraint, eg: Thus God exist and you show mathematical equations such as the the one already mentioned, Kurt Goedel's proof, etcetera. Only caveat: keep it simple and connect in a non-sequiter manner. If you show the image of two balls from one of Banach-Tarski paradox, your tagline could be something of the nature such as: Never a boring day at the classroom.
Probably a historical image in a monotone shade either a sketch of the mathematician if he is obscure or of the university or locale
Keep in mind of that if it is posted on a bullet inboard, passerby will have very short span to notice it, so it cannot be too deep to "get it"
Another idea could be "ambient"-a term in advertising, where you use the surrounding to prove a point, such as a life-size ballerina image around revolving door. Stickers on calendars, numpad on phone, mirror, trees can be ways to spread the message. Taglines like: Average person sees a tree, a number theorist sees the sequence: 1 1 2 3 5... or, Average person sees an coffee, but a topologist sees a a donut. (The last one is cliche, but for illustration purpose).
The famous book cover wittily shows and tells the theme
A Cantor one I found online that shows but does not tell the theme