Funny, the best advice for this I think is simply how my Dad taught me to speak with adults before I even entered kindergarten.
"Firm handshake. " "Stand, or sit up, straight." "Look them in the eye as you speak with them. "

You'd be amazed how many candidates don't do this. An interview is a lot like a first-date. For better or worse, humans are hard wired to form quick opinions, and then we spend the rest of the time talking ourselves into or out of what we decided in just a few moments. That first impression matters. A lot!