twirling their finger around the rim of a glass
This is something I see in movies, but can't recall ever seeing in real life. It's just not a common thing to do, and it's disgusting. Imagine all the shit you touch at a club, for example, then coating the rim of your glass with your finger.
That's actually a pretty good answer. Not in the "I'm such a virgin loser, that a simple "Hello" soils my undies" type of way, but when a complete stranger actually makes effort to say, "Hello," in a deliberate way, it can be an instant connection.
There are some good ones, in here.
The lip bite is an obvious overt one, with "I wanna fuck you" connotations.
The hair flip is a good one, because it's a hint that tells you the girl is conscious about her appearance, in your presence, even if she's not consciously flirting with you.
The eye glint and held breath were my favorites. It's that instant confirmation of attraction, without effort, and if you reciprocate the feeling, it's the easiest connection to start from. For example, you make eye contact with a girl from across the room, and she has that glint of attraction in her eyes. There's no overt physical motions or facial expressions, but the attraction is undeniable. Or, you accidentally cross paths, maybe even bumping into each other, and upon making face to face eye contact, she involuntarily pauses her breath, as she's momentarily stunned by instant attraction. Maybe you do the same. The effect can even be prolonged, if you continue to stare into each others' eyes, while you part ways. This has led to one or both persons being unable to "get the other person out of their head," until one decides to finally find the other person and continue the connection.