There is a well-known breathing technique called the 4-7-8 method.
Using this breathing method can help you fall asleep the night before a big test (like TOEFL) or other stressful things.
This method can also help you relax before giving a presentation or speaking English (if speaking English makes you nervous).
1.) Breathe in for four (4) seconds through your nose.
2.) Hold it for seven (7) seconds.
3.) Exhale strongly for eight (8) seconds.
Here is a demonstration and more about it by the famous healthy living doctor from the U.S., Dr. Andrew Weil: