Don't be serious
Or rather, don't be scared. Or don't concentrate. It doesn't matter really what I call it, as long as I keep myself from tightening, drawing in, turning inwards, closing up. Fear tightens and shortens, it makes you grab and hang on to whatever you've got, harmful as it may be. It causes you to focus on just one thing, blocking out everything else, leaving you unable to adequately address whatever changing situations may come your way.

It may sound funny, but it's true: getting out of the chair is among the most scary things I've ever done. Discovering that I was unable to let go and simply let my teacher take me out of the chair has literally sent me into a panic frenzy on more than one occasion. Afterwards, I always wondered how I'd let it come that far, what on earth made me feel that scared, and why I kept fighting it so hard. What could happen to me if I did let go? After all, it's just me and a chair, how hard could it be? There's no chasms, no jumping out of airplanes, no rollercoaster rides...except in my head.

When I notice the onset of such narrowing, for instance, in situations that involve singing (the 'I sound great in the bathroom, great during rehearsal, so why can't I sound great during performances?' issue), I physically broaden my view, simply by looking around and noticing my surroundings. And, very easy to do: I lift the corners of my mouth. It's just a tiny thing, but it makes it much easier to lengthen and widen if you do it. Give it a try, it's cool.