5 Finger breathing technique
Breathing strategies are a fantastic way to calm things down. When we take control of our breathing, it slows down the physical anxious response (so our heart rate slows, and we start to feel a little calmer) and taking control of our breathing also sends a sign to our brain that things are okay, and we've got this. So it's a win-win both physically and psychologically.
Finger breathing is a really simple exercise to do anywhere and to remember.