I heard this at a "Love and Logic" workshop several years ago. It serves as a great reminder all year long.
With the class seated on the floor, start talking about the difference in the things we say that make people feel good or that hurt them. While talking, cut a big heart out of the construction paper (whatever color!!). After it is cut out, hold it up and tell the children that each one of us starts out with a heart that is as pretty as this one. Have the children begin sharing things that a person might say that might be hurtful to someone. With each response, fold the heart (in random ways - just so it gets another fold in it for each "hurtful" sentence). Eventually the heart is crumpled in your hands. Tell the class that each time they say something hurtful to someone, they are putting a little wrinkle in that person's heart. Talk about how someone's heart might begin to look like this crumpled heart if people continue to say mean things to them.
Then have the children begin to tell things that they might say to someone to make that person feel good. With each response, UNfold one of the creases in the heart. Eventually you will have pressed out all of the "hurts"...except, of course, for the fact that the wrinkles can still be seen, even though the heart is now all the way unfolded. Talk about how we each have those wrinkles in our hearts from things that people have said to us, and that the wrinkles last forever. Stress that we want to be careful with what we say so as to not add wrinkles to anyone's heart.
We put that heart up in a high corner of the bulletin board and leave it in plain view all year. Frequently someone will mention it...that their heart is wrinkled like that one, etc. It serves as a great reminder all year long.