That's another elusive secret held by anxiety - how to calm down. For me, what works one day doesn't work on another day, but may work fine a week or two later. I have found that taking deep breaths helps. Breaths so deep that it takes focusing mentally to properly breath and manage the incoming air. It also activates the body's natural relaxation response. Usually a minute or two of this gets me past an attack. Laying down, or putting my head down on my desk at work doesn't always help either.
It's easier said than done, but do your best to manage your stress - don't add to your load if you don't have to. Stress leads to tension which tightens all your muscles like you said. It takes time, practice, and patience, but eventually you'll be able to overcome. Have faith and remember you're not alone. :)