Shaman, salespeople, hypnotists even tai chi practitioners know the fastest way to positively influence the mind is to start with the body. We start with the body because it requires less effort to make a physical change than to change your state of mind. For example most people can't change from tense to relaxed through mental effort alone.But you can easily produce a more relaxed state by making a physical change - such as going for a walk, practicing some tai chi, or by having someone massage your shoulders.
Here is one of my favourite "instant relax exercises " that can be done anywhere for an immediate calm.
Fix your gaze on something directly ahead of you without changing your focus in any way. Allow your peripheral vision to widen.
Without moving your eyes, gradually expand your vision so you can take in more and more of the space around you.
If it feels comfortable, you can take it further; just relax and allow your vision to widen...until you can sense not only the things beside you, but behind you as well.
Now you have a 'visual sense' of being centered and relaxed.