Call backs are a great way to get learners' attention and to promote unity within the class. I like to use motivational or thought-provoking call backs to add a bit of depth to my usual (and sometimes monotonous) "If you can hear me, clap three times..."
In this PDF download, you'll get seven call back posters. They are standard paper size and can be printed in black and white or in color. I laminate mine and have them hanging on a ring on my chalkboard.
I take no ownership of the quotes; they are familiar and common quotes that are mostly widely recited.