Your best bet is to get them talking about anything other than the photo shoot... work, vacation, kids etc. This gets them more relaxed and they tend to forget, or at least aren't 100% focused on the fact that they are in the uncomfortable situation of being in front of a camera. There are great books on how to pose people to hide flaws... sounds like you have a great start on getting the worst of them taken care of... I think your next step is just get them to loosen up and relax.