Be approachable, but not too obviously available. You want him to know you are receptive to his advances, but you don't want him to believe you are desperate or actively on the make.
Greet him warmly with a sweet, sincere smile. This will let him know that you are genuinely happy to see him.
When he speaks to you, look into his eyes, listen intently to every word he says and nod often to indicate that you are listening (and comprehending) what he is saying.
During conversation, touch him gently and briefly on the hand, arm or shoulder. This is a subtle, yet definite indication of interest and affection.
Laugh heartily when he tells a joke. This will let him know you appreciate his wit and share his sense of humor.
Use your observational skills to determine what his interests are. Learn everything you can about them, and encourage him to share them with you.
Learn all you can about the things he values most. Let him know that you respect the things that are important to him.
Compliment him often. Let him know how good he is at everything he does.
Be considerate of his feelings. Show him how thoughtful and considerate you are with your every action and interaction.
Tell him how important he is to you, and let him know how profusely grateful you are to have him in your life.
Let him have his space. Don't hover over him, spy on him, stalk him or act possessive toward him. Refusing to allow a man to have personal space and time is the quickest way to alienate a man and drive him out of your life forever! By all means, give him the space he needs.