This is a worthwhile thing to spend 20 minutes watching. A man called Mark Bowden giving a talk at TEDx Toronto in 2013.
I suppose there are lots of resources on the internet and in the bookshops about body language, but Marks talk is short, excellent and entertaining. It explains why you need people on your side in the first place.
If you ever have to give a talk or present something or convince someone, this will help you.