I pride myself on my 'unposed' style. (Although some of my maternity images are more posed). The trick is to make suggestions and prompts.
Some people are born posers which is not a bad thing at all, they just naturally know how to hold themselves, some are not (like me, I tense up and do a weird squint thing). So getting someone who is not used to or comfortable being in front of the camera looking relaxed is hard, if you go ahead and pose them it will just look forced so I use a 'prompting' approach.
I'll ask a series of questions to get a good response, tell them to think of a certain thing (e.g. for a maternity shoot 'close your eyes and concentrate on feeling your baby moving inside',). This works so well and gets me the most amazing REAL and emotive photos.