This kind of self awareness would be a good starting point to fix the problem if you decided it was important enough to you. There are various things you can do which plausibly claim to improve body awareness (I've heard the Alexander Technique mentioned around here though I don't know anything about it myself) and good body language can be learned to some extent. Even if you don't think it's worth the effort to work on these things however, if you go to a good professional portrait photographer they should be able to help you address these kinds of problems and get some good pictures. Portrait photography isn't my main area of interest but I've read some books that cover the basics and they generally talk about techniques for getting the client relaxed and comfortable in order to minimize the effects of awkward body language and about things you can tell a person to do that will help them position themselves in a way that will produce good photos.