The meditation app Calm was helpful for me. Meditation has common misconceptions around it, but it's really a tool that helps a person observe their self. That has a calming effect (resets current stress level) and a preventive effect (helps me understand when I'm getting stressed).
There's lots of other meditation apps, so shop around as needed.
Talk therapy is helpful, since it forces you to address the underlying thought processes behind the stress response in an environment that is outside of said process, with someone who can point you to alternate thoughts and beliefs.
In the pure physical response, you can practice breathing exercises or distract the body with another stimulus. A friend of mine who has very high general anxiety uses their phone. Another friend will grab a handful of ice (in a bag to avoid mess) and hold it in their hands until it melts.
There's lots of other meditation apps, so shop around as needed.