No more than two or three type faces. They should be legible, not too many
capital letters, and not overly ornate.
Titles should be well thought out to capture the audience. Good titles and
poor titles cut out from magazine pages can help them understand how important
the initial message is.
The picture should use the same colors as the title. Pictures enhance the
message and make the audience want to stop and read the poster. Movie posters
taken from video stores make an excellent lesson on composition, size
relationships, rhythm, and color.
This is off the top of my head, but I was a commercial artist for eight years
before I became a teacher.