I've certainly read Yalom talking about this problem, and he mentioned that he thought his therapist may have had the same problem with him - his therapist always sat in an uncomfy wooden chair!

I've found not yawning to be a problem. It does not imply I'm sleepy, just the stuffy consulting rooms. I thought I'd mastered the art of the closed mouth "hidden" yawn, until one of my clients told me to just yawn if I wanted to!....which actually opened the way for a great session about my reaction to my client and how they perceived I thought about them.

I find a museli bar pre-session and a big glass of ice cold water during the session helps!