Read and learn the basics of posing theory (there's more to posing than just the poses); learn about WHY certain things are done, and then get a willing vict... helper and practice. That's really all it comes down to. Learning the theory and becoming comfortable with the idea of directing someone, and IMO, the best (and only) way to get good at it is practice. Bear in mind too, that every photographer has a different approach to this, and what works for me, and how I direct models is going to be different from how you do it; it's important to develop your own style and find out what works for you.
Critical points though... ALWAYS be professional. If you need to touch a model, ask them if you can, tell them you're going fix their hair (or whatever), when referring to parts of the body, use adult language. Breasts are breasts, not boobs, etc, etc.. maintain as much contact as you can. DO NOT hide behind your camera. If you talk to the model lower the camera and look at him/her.