Good posture signals to your audience that you want to convey things to them that you are confident to discuss. Good posture for meetings and presentations is not a military posture, which can look anything but relaxed. It’s a prideful posture. Chest out, shoulders slightly back. Head held high. Big smile.
If you’re seated, sit closer to the edge of the seat, lean in towards the table but don’t slouch.
Now you got them right where you want them.