Write them, especially when you are out of town on a trip. Let them know how you are doing and that you are thinking of them. Share personally what God is teaching you. Don't be afraid to share some of your own needs. If someone loves you he will also love your message.
Friendship opens the door for teaching and training others.
Friendship follow-up will never work without it. Begin today to build relationships with young Christians. They need you and you will be amazed at how much you will enjoy being their servant.