I have tried this method and found it effective: http://www.telegraph.co.uk/news/science/science-news/11585755/Simple-4-7-8-breathing-trick-can-induce-sleep-in-60-seconds.html
Essentially, it is just doing deep breathing for 4s, then 7s, 8s, and 9s. It helps to slow down the heart rate and will naturally relax the body and promotes sleep.
I think it probably also helps prevent the mind from being distracted by other thoughts and helps one fall asleep.
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