We don't generally think of hair as a part of the body we can control, but you really can! If you are shooting a subject with long hair, then bad hair is going to be the first thing anyone notices about your photo. There are no rules as to what looks "best" across the board. Everyone will look different with their hair a different way.
Let's assume you're doing a basic portrait session without makeup artists and hair stylists. The first thing to remember is that hair sitting on the shoulders looks terrible. If the hair sits on their shoulders, then it looks wild and you need to do something with it. There are five different things that they can do with their hair.
Hair on the shoulders (#1) should be avoided at all costs. All of the other hair positions have their place depending on your model and the look you are trying to achieve. The reason I shot the hair on both sides (#4 and #5) is because the natural part in a person's hair will lend to one side looking better than the other.
Generally you want their part facing the camera so more of their face is included. For this tutorial, I chose hair up (#6) so we can more easily see the posing instructions without distraction. Many women see ponytails as the "day-off" hair style, but it actually lends itself very nicely in portraits and headshots since you have clean view of their face.