Your body language is louder than your words! Even for those people who are really loud.
You look at him, he looks at you, you both smile, then he walks over to say hello. OMG What just happened!? When you make contact like this, it's a common language that you need to be aware of. Eye contact, and smiling are invitations; you just told them they are allowed to come talk to you. What if your shy and can't keep the eye contact, or your smile gets lost in your nervous pressed lips? Well, some guys will see past this. It helps if your able to make some double takes when you look at them. If you can't do it though, then your going to have to find a way to practice.
But fear not! There are other things you can do as well!
Positioning your pelvis. Basically, you need to point your pelvis in the direction of the person who's attention your focused on. This means you can turn your shoulders away in that slightly shy move, you can even turn your head away; but half of your body is still facing the other person, so they know your attention is on them, or that its going to return to them.
Once upon a time, five of my friends all decided to use this method combined with smiling faces and sparkling eyes on a cashier at the grocery store. They never shifted their hips, or took their eyes off him. His grin was ear to ear, I thought his head was going to explode !
Let's say your not very good at noticing people. Your head gets all fuzzy in a group of people till one singles you out. This will make it kinda hard to focus your eyes, or your pelvis, to let anyone know what your up to. Best thing you can do (fuzzy head or not), before you even enter the room, stop in the doorway. Let your friends walk in ahead of you, and take a "look" around the room, as if your trying to find someone (even though you know where your friends are). Your little pause, gives people time to notice you, and the doorway creates a kind of frame around you, like a portrait. It doesn't always turn every head in the room, but someone will notice you. It's up to you from that point to decide what you will do to keep their attention.