Rather than discussing this in detail, my view is you were taught everything you need to know at school. Stand up straight, make eye contact, speak clearly, sit with an upright posture, and listen attentively. In fact what you are subliminally communicating, and what people are subconsciously evaluating you for, is whether you are healthy, vigorous and a “good student”; willing and able to participate in a project with them. Nothing has changed since cavemen formed hunting parties, or football teams are picked in school yards, or you went for your first job interview; the healthy, energetic, enthusiastic get picked, the weak and morose get left out. Do you want to be on the customer’s team?
If you really want to study body language you can read Alan Pease, who has written several books on the topic.
The only other scenario where you need to worry about body language is when you are doing a presentation to a large audience. The basics remain the same; stand up straight, speak clearly (with a microphone if required in a large hall) and make eye contact around the audience. This is a slightly “false” behaviour the first couple of times you do it, but important if you want to engage an audience. Do not talk to the floor, or worse the screen with your back to the audience; talk to and make eye contact with specific people around the hall. Look at the seating arrangements before you present, split the auditorium into 6 or 8 sectors and rotate your attention through each of them in turn. You do not need to talk to the same individual in each sector as you rotate your attention, but always talk to someone specifically. This helps keeps the entire audience engaged (as long as your presentation is of some interest!!!). The other key to presentations is self-confidence; but at a specific moment, the start of the presentation. Even if I have complete mastery of the content of a presentation, I always rehearse the first 30-45 seconds of my presentation (in a mirror, if possible). I introduce myself, my topic, the agenda of the presentation and the first slide. After this remarkably short period of observing you, the audience will have already judged you as a presenter and you can make a “normal” level of stumbles after that without losing your audience. Just get the first 45 seconds word perfect and project confidence.