I never knew either. So what I started doing was sticking my hand out for a handshake and then say something about it in a goofy sort of way. Like "So what do we do here? Handshake? Hug? Tip each other?" I think it lightens the tension, and if she giggles it just seems to me it would be easier for her to say whatever she thinks is appropriate, whether it is the handshake, or the hug, or whatever. But that's my shtick.

For the record sometimes I say something totally unfunny because I just come up with it on the spot. Then I just laugh at myself.