The "trick" is to let you mind wander without intentionally thinking about anything, but just letting thoughts float on by without getting attached to any one of them. Just notice all the different things your mind is bringing up, and let it happen, without grabbing hold of any one thought.
My big problem is that I have a strong inner voice that is explaining or narrating almost all that I do. This voice if focusing my thoughts, so I need to stop talking inside my head for this to work. I kind of just need to shut up and watch my thoughts instead :)
I force myself to. I just simulate what happens when I really start falling asleep. I create random thoughts and I chain them as fast as possible.
Example:
If I break the ice with my hammer the calendar will fall on monday, but I have to escape from a huge mountain today, etc... Works super well for me. If I really need to sleep I'll use that technique, if I'm not sleepy I can do that for like 20 minutes, I have to focus not to think about something and just think about those random and nonsense stuff and I will fall asleep eventually.
I've noticed it is often preceded by having some thought, then 5 seconds later realizing that the thought I just had makes absolutely no sense. I guess that's the reality testing switching off.
The sleep is worth the practice :)