I kinda have a little routine I like doing for fun.
I lock eyes for a few seconds as we're walking and whilst she's still looking I quickly check her out, like down the body and back up again. I want her to notice me checking her out but as if it's unintentional, as if i didn't mean to. As we've already been staring at each other I can assume she's slightly interested and so my subtle checking her out isn't creepy. Then i'll lock eyes again, smile and bite the corner of my lip. They always smile back.
But to be honest if you've had the initial staring each other out bit then they'll smile back anyway
I just like the whole thing cos it's fun and kinda cheeky without seeming to be intentional.
I go to university so although I see lots of different girls each day, I also see a lot of similar faces.
Which is also why I have so much time to try different ways to stare out girls... .
Check out propinquity in relation to seeing the same faces around. Basically if someone keeps seeing the same person around, not as in consciously recognising them but subconsciously when you scan a room, when they finally talk to them they feel greater comfort as they subconsciously don't see them as a stranger as they're sorta used to seeing their face. Something like that anyway
http://en.wikipedia.org/wiki/Propinquity