Most Helpful Girl
Smile. I always tell people to make eye contact to let a guy know but personally I get really self conscious and shy and look away, but thats me personally, a lot of girls will look away as a kind of "not interested" (but guys do the same). Thats if you don't know each other very well.
If I know him... I don't know flirting I suppose. Apparently I flirt with people but I've never known I was doing it, I think it's just a natural reaction you'll start flirting without even trying!