I like when the girl does that shy stuff but after she's confident in me. idd like to stare longer into her eyes and smile to each other. gives me a feeling that were attracted each other. if a girl is close to a boy and they are about to kiss. I just stare her eyes and check if she wants and actually the girl goes for it.
It depends on who the guy is and what you think about him. Think he's cute? Look away quickly once you notice him, acting like he caught you looking at him, but look back at him, like you couldn't resist one more look. Give him a small smile, blush, or tuck your hair behind your ear, something like that. Guys love that girly stuff. If you don't want to encourage him, just look away and ignore him.