The 3 Minute Breathing Space can be helpful as a quick grounding practice and to calm down.

This mini practice aims to help you step out of the automatic pilot and reconnect with the present moment experience. It can be very useful when you feel you are losing perspective and negative thought patterns are taking over that tend to push you into a downward spiral of emotional reactivity.  

Step 1: Wide focus of attention 
Step 2: Narrow your focus of attention by attending only to the breath
Step 3: Expand your awareness out again to whole-body breathing

Here is a vieo that will talk you through it: