Essentially do not think about things that has happened; It just gets you annoyed with regrets that can not be changed. Do not think about the future; You will get excited, anxious, annoyed and too worked up to sleep. Also covers sports, politics, relationship, technology etc.
Think about things that do not affect you as a dream. A fantasy world etc. That has nothing to do with you personally so that it is just calming but distracts you enough until you are soon asleep.
[1] https://blog.flurdy.com/2010/02/excellent-tip-on-how-to-go-t...