It's best to break rules AFTER you know them.
There are exceptions to every rule, we all know this... But what purpose does a rule have for existing? It's an instruction - based on an average - that regularly gets good results. So today I am going to do my best to teach you the general rules of posing, with a focus on the torso, shoulders, neck, head, and eyes.
After you have chosen your location, figured out your camera, and flash settings. You will decide how to place your subject in the scene; sitting, standing, leaning, crouching, laying, climbing, etc. I like to decide which way my subject's body will be angled based on the scene and framing. Then we can begin posing:
1 Do not shoot straight-on towards a subject's chest. This gives the impression of them being broad or overweight. You will want to have one shoulder further from the camera than the other shoulder.
2 For Men, you should tilt the top of their head towards the shoulder that is further from the camera. For women, do the opposite.
3 Aim the subject's nose between your lens and the main light source, this will create what is known as short-loop-lighting. The effect makes their face appear narrower, by placing more visual emphasis on the side of the face that is revealing less physical surface to the camera.
4 Have the subject raise or lower their chin (face) as necessary, you don't want to see up their nose, you don't want to see lots of their neck, you want to avoid having them lower their face too much though, as this tends to scrunch up any neck-fat and create a double-chin. You'll know you have gone too low if you can see the white of their eyes, directly below the iris.
5 If necessary, you can instruct them to "turtle" their neck straight forward towards the camera. Be aware that most people have difficulty keeping the shoulders, tilt, turn, and head-height consistent while doing this - so you will have to re-adjust as necessary - sometimes making this maneuver not worthwhile.
6 As people try to manipulate their necks in unfamiliar ways, their necks and shoulders tend to tense. So take a look and be sure the shoulders are down and not rounded forward. If the neck is showing unattractive tendons, usually a deep breath will relax them.
7 The eyes should point at the lens OR follow the direction their nose is pointing, watch for droopy-eyelids, and if that's the case, try having your subject look slightly above the lens. This should force their eyes to open a bit wider, without being noticeable.
If there is any confusion about the terms above, make sure to check out Posing 101