It is a matter of passive concentration.
When you close your eyes you begin to see what is known as hypnogogic imagery; this is what leads you to the dreamworld. When your mind is too active it cannot go with the flow.
The first thing you need to do is to stop thinking that you can't fall asleep. As soon as your head hits the pillow, close your eyes and watch the images that you see without attempting to control them, but also do not let your mind wander into worries or other matters. If you do this immediately upon going to bed it will work much easier. If however, you decide to "think about things for a while" before you try to fall asleep it will be much harder to stop these thoughts, and your fear of not sleeping will keep you from sleeping. Read that last sentence a few times and let it really soak in. If you KNOW that you will have no trouble falling asleep, you will not; period.
If your mind just cannot be calmed down, then usually a forced concentration on your breath or one other thing will help to focus the mind enough that you can try again, but such concentration takes more practice than most people are willing to put in. After years of practice, I can focus my mind on one thing at will, and all else is nothing. I can even go into a sort of "trance" before sleep, where my mind is cut off from sense impressions entirely. No joke though, insomnia is a problem of an out-of-control mind, and you will benefit greatly in learning to focus on one point.
Drugs are no answer in my view. The easy way is always a waste of time.