Read books that explain and encourage friendship. There are a wide variety of books that can be used for this lesson. Some really great friendship books for preschoolers includes "Rainbow Fish" by Marcus Pfister and "The Giving Tree" by Shel Silverstein. These books demonstrate the good qualities a friend should have. They are also beautifully illustrated stories and will hold any preschooler's attention.
Lead a group discussion on the book. As soon as the book has been read, discuss what happened in the book with your students. Ask them questions to reinforce what they just heard. For example, if you read "Rainbow Fish," you can ask the children what Rainbow Fish did to be a good friend to the other fish. If you read "The Giving Tree", you can ask the students what kinds of things did the tree give the boy, and why they think it is important to share with their friends.
Complete a craft to reinforce the story that was read and the lesson learned. Doing a craft will remind the children of the story and reinforce the lesson. Preschoolers do not often remember or understand things the first time they hear it. Repetition and reinforcement is very important.
For the "Rainbow Fish" story, you can have a large fish cut out of paper on the wall. Then give each child a piece of paper with a half-circle drawn on it. Have the children cut out the half circle, and then draw a picture of themselves being a good friend on the half circle. Encourage them to decorate their picture with lots of bright colors. When all the students are finished, you can tape the half circles onto the large fish to make up the scales of the rainbow fish.
For the book "The Giving Tree," you can do a similar project, but have the children draw on leaves and tape them to a large tree attached to the wall.
Later that same day, after the craft is complete, reread the book that was initially read to reiterate and reinforce the lesson being learned.