Always maintain eye contact.
No more than a 10 degree nod. You are acknowledging, not looking for birds.
A slight eyebrow raise with a corresponding downturn of the corners of the mouth.is acceptable.
Never smile, you are not looking for a date.
Nod only once, you are a man, not a bobble-head.
No words. Done properly the up-nod speaks volumes.
Never up-nod a woman, they just don't understand.
THE SILENT SIGN OF RESPECT