It’s an old adage that a billboard has 7 seconds or less to make its point. That’s a good guideline for posters.
Just the top of the (inverted) pyramid
There’s a page in the toolkit dedicated to the inverted pyramid, but basically whatever you’re advertising in a poster needs to be boiled down to the absolute essentials. That is, who, what, when, where, and how much. It’s the proverbial elevator speech in poster form.
Consider the places where posters typically appear: Public billboards, the windows of businesses, and probably any available vertical surface where the target audience is likely to be. When your eyes pass over a poster, how much time do you give it? Two seconds, if that? Your potential audience is the same: Busy and distracted. They’re exposed to 2,000+ messages per day. You need to respect that.
As always, the design should serve the content — not the other way around. Put that who/what/where/when/how much stuff front and center. If it’s a seminar with a guest speaker, you probably will want a high-res photo of the speaker on the poster, but unless your speaker is Neil deGrasse Tyson or Barbra Streisand, the photo isn’t going to sell it. For a poster, you’re trying to draw someone’s physical gaze. That means brighter colors, gigantic words and a clear message about why it’s essential. There’s a reason why posters advertising bands are sort of “out there.” It’s so your eyes linger on it a few seconds longer than the others.
Usually, you’re going to have to lead with either the speaker’s name or the name of the presentation. There are valid political reasons for doing so, and for the poster to look professional. Ultimately, it boils down to what the poster will compete with. If it’s a summer event on a college campus, maybe you don’t have to get too fancy. If it’s fall on a college campus, you will.