I have trouble staying asleep, wake up for as much as 2 hours with so much going on to consider. What I do when I realize I HAVE to fall asleep is an old exercise from yoga, or other relaxation training. Starting in your toes, you take yourself down to the toes and turn of the lights, as if the body were a building and you, a night watch man or woman in this case, amble through from toes, to instep to heel to ankle and so on up through pelvis and torso to the heart level and then down the arms, up into the head, leaving a light burning in the heart area. It does work.
Otherwise, I have learned to notice tense muscle areas and change them as I go through the whole body, imagining or conveying to that centre of nervous activity that it needs to let go, feel heavy and let the bed/pillow hold the WHOLE weight of that part of you. This reminds me of another new thread, posted by Placematt, 'What is safety', this past Monday morning. It seems like a cross references with your question, nicolelalonde, as this thing of relaxing muscles relates to whatever is making you/us feel unsafe and tense.
A dance teacher used to say without identifying the feeling of relaxed muscles, she would have spent the night with her head never touching the pillow.