One strange but great tip we have for women (sorry guys, I’ll get you next time!) is to wear a sleeveless dress for their headshot. It might sound a bit to scantily clad for a first impression of a professional as the temptation to look professional is derived typically from a tailored suit.
Rest your judgements to the side, as psychologically it has been proven that the sight of other human skin does make us feel good, and somehow more connected to one another. You get the perfect taste of humanity in your headshot since you don’t need to show the entire arm, but just enough. Test it out by imagining this photo with the shoulders covered and decide for yourself which is stronger!