Repeated contact...at least three separate verbal or non-verbal clues need to be given. Why? The first time he's going to look around and make sure it's really him that you are flirting with. The second, he knows it's him and he gets flushed and pleased. (At this point he'll probably walk by you and at least smile..he's checking you out a little more.) The third time you can express interest-by introducing yourself, or commenting on his tie, or waving from across the room. Now he knows you are open to meeting and it will be a cinch.