Summarize this article:

Smiling is the easiest way to show someone you’re interested. If you see him looking at you, give him a quick smile. If he’s thinking about kissing you, this might encourage him to move closer to you. Eye contact is a great way to flirt, since it creates an instant connection between two people. Flirt with your eyes by looking at him then looking away, then looking back for a longer gaze. When people are feeling flirtatious, they often signal this unconsciously by playing with their hair or touching their face. Send this signal intentionally by resting your hand on your cheek, wrapping a piece of hair around your fingers, or lightly touching your throat. This is especially effective if you’re talking about something you’re passionate about, since it seems like a very natural movement. If he smiles or leans towards you, you can leave your hand where it is. If he seems uncomfortable with the touch, pull your hand away. Chances are that he’s just as nervous as you are, so boost his confidence by laughing at his jokes or listening intently to his stories. Be sincere, though, since he’ll be able to pick up on fake laughter.

Summary:
Smile at him. Look into his eyes flirtatiously. Touch your face or twirl your hair around your fingers. Rest your hand on his arm while you’re talking to him. Laugh at his jokes.