Sit still.
Make your setup a more calming area.
Have food and drink on hand.
Remove all distractions form the game.
Make sure that your gaming area is lit enough so you dont fall asleep from this awesome guide.

First get in a comfortable position such as standing straight or sitting in a relaxed position.
Now you gotta be ready to breathe
Breathing is key.
Make sure your not in a cramped space because that's no fun breathing in.
Now relax your body one muscle at a time, tightening then release. Start from your head and make your way down.

The Exercise Edit
Stop by clearing your mind of thoughts.
Close your eyes (if you are mid game this is not necessary)
Focus on one part of your body.
Breathe in for 4 seconds then wait for 4 seconds
breathe out for 5 seconds then wait for 5 seconds
Repeat the breathing steps until the end.
while breathing focus on specific parts of your body so as to ease your mind
Some may want to skip genitals as to skip any chance of lustful thoughts arising.
after focusing on different body parts then
Now focus on your heartbeat
After 20 seconds of just focusing on your heartbeat open your eyes.

this can be used for more than just keeping your cool when gaming.
This can be used whenever you need to keep your cool or improve your mental state.
You can always pick a few steps that work best for you and use those.