Break eye contact. Sometimes things get so astounding you just need to stare at the table for a minute to reevaluate your place in the universe.
Overshare. Doesn't matter if it's relevant: A quick story about an event that changed your life will make it seem like you're in really deep.
Swivel your head. Nodding "yes" means "I get it, shut up." But a slow "no" means "I can't believe the fascinating things you're telling me!"
Lean back in your chair. This says you're totally relaxed. You're ready to hear some secrets. Don't lean back too far and don't get too comfortable -- it won't help to fall asleep.
Communicate by saying nothing. Words like "interesting," "fascinating," "wow," "incredible" say you are listening but its meanings are neutral enough not to make yourself look clumsy. If asked a question, say "I agree with you." Watch out, though. It's a careful balance. Don't say too much or you're just going to egg them on.
Make Light Contact. We're not talking a bear hug here. We're talking about a touch on the shoulder or elbow, a firm handshake. This shows you are engaging and willing to hear all about... you know, whatever.
Repeat "salient" statements back to the speaker. "So, you're saying that no matter how good your presentation is, Corporate won't notice because Wilson sucks all the air out of the room." Picking a particular statement and paraphrasing it back to the complainer suggests commiseration, and everyone loves company in the dungeon.