The only technique I know of deals with anxiety. Have the person experiencing anxiety sit down and sit down next to that person directly facing him or her. Look at that person and have that person look at you. Tell that person to breathe with you Take deep, slow breaths.
I have never practiced this technique. This was taught at the Citizen's Fire Academy.
Jeanette Isabelle
Might help, might not. People have different respiratory profiles in panic/anxiety attacks. Probably the majority will feel a bit calmer after a little (a little!) slow, deep breathing. Others, however, will feel worse. In a crisis situation you don't really have time to carefully explore this, so I'd recommend that you might try it briefly - but if it doesn't help in a minute or two (or if it seems to make the person more uncomfortable) don't persist.
My experience is that what helps the most universally is to just let them talk about what their physical sensations actually are. This "processing" seems to lighten the discomfort pretty reliably. I've never seen it have an adverse response, and I certainly have seen adverse responses to any of the typical breathing techniques (Benson Breathing, Diaphragmatic breathing, etc.).
I suggested trying the above slow-deep breathing briefly because even if the person feels worse you haven't done any harm: they'll usually just stop complying with your instructions.
The "face to face" helps some... makes others feel worse. Again: try it, but pay attention. If the person feels worse, perhaps have them shut their eyes.
The bottom line is to not wed yourself to a particular tool: watch, and change what you're doing if it isn't working.