It's not enough to quickly catch the eye of the guy you're interested in, then look away.
However, you also don't want to stare him down and make yourself look like a stalker. Instead, catch his eye and hold his gaze for a few seconds before looking away, with a smile on your face. Wait a few seconds and look again. At this point, he'll probably know that you're interested. Give him a quick wink and gesture with your head that you want him to come over. If there's an empty seat next to you, pat it with your hand while looking at him to send the message that you'd like him to sit with you.
You don't want a guy getting mixed signals, so shy flirting is out.
Be direct and make it clear that you want to get to know him. If you're with a group of other girls, separate yourself from them. Guys can feel uncomfortable approaching a girl when she's surrounded by her friends. Go up to him and introduce yourself. A simple "hi" is usually enough to get a conversation started. A lot of guys will be flattered that you approach them instead of the other way around. If he's with a group of his friends, ask him if you can speak to him alone. He'll enjoy the attention of being singled out by a girl.
Lean toward a guy when you're talking to show your interest.
Listen to what he's saying and ask him questions about his life. Ask about his likes and dislikes in music or culture or what he likes to do for enjoyment. This shows that you want to know more about him. If he asks you questions about yourself, answer naturally and honestly. A smile makes you seem friendly and approachable, making the conversation easier. Laugh at what you find funny, but don't be fake about it. Lightly reach out and touch his arm or his leg during conversation, but only if you feel comfortable doing so.