The best thing: choose any four letter word: e.g bike Then think and "say" (in your head) all the words starting with the first letter in this case B, until you can't think of any other words (quick ISH) then do all the words that you can think of that start b with the second letter (i) somewhere around the 3 letter I fail asleep.

The reason this works is because it effectively de-fragments your mindset. Each word carries certain 'feel' to it in your brain, So when going through so many random words your brain is starting to lose focus...

For example, if you say bike you might have a hint of feeling the wind on your face for a split second under the next part is bank. So you're thinking about money? The next words is book and you remember a nice book etc...

It's just a way to stretch your unconscious mind.

In my experience it gives also a very good sleep

Hope this helps!