Ha, yeah, I know what you mean.
My solution to "stiff" models is having them stand up, literally shake their body, arms legs, kind of like a on the spot trot (jog), then sit back down. It gives them a WAY more natural and relaxed look.
Secondly, once the body is in position, the head can be treated as a whole other separate entity,
To position the direction of the head:
Look through your viewfinder at your model, then extend your hand and tell them to look at and follow your hand until you are satisfied with the head position.
Then, without them moving there head, do the same thing with their eyes. Usually you will have to tell them to "look over there" as they eyes are better when looking off into the distance.
Example to illustrate what I mean with head / eye positionning:
(Older picture, I wanted to see what I could capture in camera without editing. Mostly a success except for one, er, cut. If you don't see it, lets keep it that way

)
Also bare in mind that there is a certain sweet spot with head positionning when doing portraits. NEVER have your head going straight into the camera, always tilt the nose a little down and to the side, like this: