I have very consciously been trying to analyze my embouchure when I feel it is on. Ligther, snappier highend and stacattos, better accuracy, more fluid legato, etc. I have been inhaling "AHHH" and notice that after that, by thinking about lifting my cheeks (squinting slightly) stuff suddenly becomes much cleaner. Has anyone else noticed this?

There was a week in my horn career when I found my playing quickly becoming tighter and more tense, tounging airballs left and right. Looking in the mirror, I realized that when I went to attack a note, I was tensing up my cheeks and squinting. After a very short time of consciously making an effort to relax my face, these issues were resolved.

I am inclined to say that the more relaxed your body is (except of course for the muscles directly making the sound) the better. However, everyone is different.

I'm inclined to agree with DanShapiro. Excess tension will contribute to fatigue.

Valerie