Mindful breathing for reducing stress/anxiety and calming the nervous system.
The 4-7-8 exercise:
Inhale through your nose till the count of 4.
Hold your breath till the count of 7.
Exhale through your mouth till the count of 8.
Repeat 3 more times for a total of four breaths.
Start with 4 breaths, with regular practice you can increase up to 8 breaths.
Use this technique as often as you like!