We basically, as has been stated, don't.
The only thing I can think of that I do is to be in her line of vision and trying to stand somewhat near to her; no where near next to her but somewhere if she were to turn her head a little she would see me (if I couldn't be in her line of vision or if I were too nervous to be in the line of vision).
And we don't give eye contact unless for a split second, if you catch us red handed. When you aren't looking at us, we are looking at you (yeah, sounds kinda creepy).
Basically we do everything that shy girls do ...and it's absolute hell being shy.
We are constantly waiting for you to approach us or say something, anything, to start the conversation (mind you - you have to keep it going also & we would love it if you were to ask us for our numbers).
Mind you, if the guy isn't interested he's also not likely to keep the conversation going, so it's a very thin line between being interested and not interested; hopefully you can read the small signals correctly. And if we aren't interested we'll still give out our numbers, even though we don't want to (we are just being polite) so you'll have to watch his body language closely.