There are some different styles, and a variety of poses, but for classic portraiture, it's all about the face. Posing the body helps the overall composition and visual interest, but if you forget all else, make sure the face, and particularly the eyes are captured well.

• No football shoulders. Start with the subject, whether seated or standing, point their body (e.g. belly and hips) at an angle, e.g. 45 degrees, to the camera.
• Tall back. Not rigid, but no slouching.
• For standing, have the subject put their weight on one leg and "pop" other out in front, which is to say it is slightly bent.
• No short sleeves, or short pants. The eyes are attracted to bare skin, and for a portrait, it's about the face.
• No disembodied body parts. If arms are crossed, make the fingers don't just come out of nowhere, bring the whole hand out and have it holding the arm.
• Tilt the head. For men, tilt the head slightly toward the lower shoulder, for women tilt the head toward the higher shoulder.
• For women, the mermaid pose is good if seated. Whether on the floor or seated in a chair, have them rest on one hip and bring both legs together to one side. The legs create a leading line.
• In general, tummy away from the light, nose toward the light, eyes toward the camera.
• Don't turn the head so much that the nose breaks the cheek line.
• The eyes should show whites on both sides. You don't want the iris/pupil all on one side.