Well, I think the position and shape of the neck relative to his shoulders is what is throwing this picture off. Look at the neck--it is curving on the left side (his right side) which is in itself unnatural.
On a real person--if someone were to turn like that, the neck would form a very straight line do to being 'pulled' to the side like that. Start from where the neck connects to the base of the head and draw out a straight line (not too narrow) to where the clavicle would be. Then go back up about half way (where the shoulder is already connecting to the neck) and draw out the dip of the shoulder--making sure to 'dip back up' at the end of the shoulder where the arm connects to show the strain on the muscles.
In this position, the arm holding the glass (at least that is what I think it will be ^_^) should drop down--and be partially blocked from view by the side of the man's body. It is very hard to hold your arm like that in this position--so I think to make it seem more natural, you should draw it almost disappearing behind his chest, making sure not to loose the roundness of his shoulder. You might also want to raise the other shoulder (the arm that is holding him/her) a bit too, just to make it feel like he is 'reaching around her/his back'.
Make sure to also draw in the bulge of this chest and stomach in order to show it overlaps his arm ^_^.
Here is my version, to show you what I am talking about. I hope this helps. I'm not saying 'draw it like this', but it is a good reference to my 'solution' to your problem, which you asked for! I can't wait to see the finished version to this drawing ^_^.
http://img70.imageshack.us/img70/70/posein4.jpg 
