A short, common, and easy-to-remember phrase is a saying. When class ends right before your turn to give a speech, you might use the saying, "Saved by the bell!"
A saying is basically the same as an aphorism, an adage, or a proverb, usually offering some kind of wisdom. If you accidentally break your sister's favorite teacup, you might think of the saying "To err is human, to forgive divine," which means that everyone makes mistakes but only truly remarkable people are able to forgive these mistakes. You should probably also remind yourself of the saying, "Honesty is the best policy."