Smiling is the best way to let someone know you are interested in him.
It lets the guy know you are approachable and you are interested in having him introduce himself. It also shows that you won't reject him.
Make eye contact.
He will notice that you are interested in him if you make eye contact and hold it longer than one second. You can use the double tactic too - you glance over at him once combined with a friendly smile, followed by another glance. Don't stare or be too sultry because you don't want to give him the wrong impression; you're nuts or you're easy. If he doesn't approach you after that then a) he is stupid because he doesn't know that you are flirting or b) he is not interested.
Waving is a great way to show you are interested without being obvious.
A little wave to catch someone's eye is a great subtle way to say hello.
Winking is also a subtle tactic to flirt with someone.
It shows that you are approachable and interested in speaking to him.
Touching your skin shows that you are interested.
A little caress on the shoulder goes a long way.
Better yet, touching him is a great way to show interest in him.
Brush his arm or lightly touch his hand during the conversation. You can also touch him on the small of his back. Yet, if he pulls away, don't touch him again.