Always maintain eye contact.
Just a slight a 5 degree
nod. You are acknowledging, not always agreeing.
A slight eyebrow raise with a corresponding downturn of the corners of the mouth.is appropriate.
Never smile, it's creepy.
Nod only once, you're not a bobble-head.
No words, the up-nod speaks volumes.
Never up-nod a woman, they just don't understand.