Here are the basics, largely taken from Transcendental meditation
-Find quiet comfortable environment where you feel safe.
-Sit comfortably and close your eyes
-Relax your muscles starting with feet and working up to face.
-Become aware of breathing, as you exhale say to yourself a one or two syllable phrase, often called a mantra. It should contain soft consonants. For instance, you might say "one" or "om" (as the budhist of do) or "amen" (as christians often end their prayers).
The key to the technique is to focus on the breathing and let your thoughts wash over you, and maintain a passive attitude. One person said the key is being in the moment. We spend so much of our lives thinking and worrying about the future or the past, and this process allows our brain to take a break. One technique I was told about was to count on each breath and don't have any thoughts, once you have a thought start over. However, part of the technique I have learned says not to worry if you do start to think and loose focus on your manta, just gently start back up. The point is relaxation, not discipline or concentration, but with practice you become disciplined.
When I first started, I noticed sometimes there were things that would come up that I just couldn't let go off... For instance, I would think, I need to check the mail, I need to pay my bills. I need to post about meditation on my blog. I need to do this and that. Some times I would even get up and do it. I now have a list of stuff I usually check that is done so that I'm not distracted. So it's added some routine to my night. However, there will always be something that you need to do so at some point you just have to let it go.
It's recommended to do it twice a day for 15-30 minutes. I usually do it once at night, often it will put me to sleep. That has been part of my problem with sleep, I can't cut off my brain, I toss and turn and worry about stuff.
Dr. Herbert Benson, has done research that shows this invokes what they call the relaxation response. It is a hypothalamic response that is the converse of adrenaline. It lowers blood pressure, improves sleep and has other health benefits.
(I got a lot of this info from Ray Kurzweil's book Fantastic Voyage)