I discovered this many years ago and use it when needed. It's simple, drug-free and works every time, for me.
Hope it helps the insomniacs.

Most sleeplessness is a result of restlessness or an inability to relax your mind. This method is the opposite, it actually requires you to work hard mentally, with great discipline. If you can hold your mind on topic for a few minutes, without even noticing...you'll be asleep!

Here's how:
While lying in bed, concentrate lightly on your thoughts...looking for the mental images that are always flowing through your mind.
After a few seconds you'll start seeing shapes, colors, whatever. Doesn't matter what they are, start small and let them grow.
Don't attach to the content of the images, simply watch them, let them come and go as they will. Enjoy them, this's what it's all about.
So that's it. That's all it takes. You won't notice, you'll just be asleep.

If you do this on a regular night it only takes a few seconds to fall asleep. But when your mind won't shut off, you have to keep dragging it off the tangents it wants to go on and back to watching the images.
Do it ruthlessly. Focus hard. Be brutal.
If you can hold your mind on the images even for a few minutes continuously, you're there.

I think the way this works is there's always a stream of images flowing though our minds. We don't see it during waking hours when the external images have to take precedence, but at night we lower our consciousness into these images, which we call the dream-state.
This technique uses the images to put your mind directly into the dream state, and hence sleep, through conscious intent rather than leaving it to the vagaries of relaxation.


A side note: I found this method when I was trying to consciously enter this dream state, trying to experience the "hallucinations" of being high without the drugs. It was tremendously annoying: just as I was getting there, seeing the amazing creative, spectacular images.....zzzzzz.
It was only months later that I realized I'd inadvertently found a way to get to sleep! Damn!