Meditation is a state of deep physical relaxation combined with acute mental alertness, and there are many ways to achieve this state. Almost every religion incorporates meditative practices such as praying or chanting, and you might be able to find one in your belief system. Other purely physiological techniques involve sitting and focusing on something that will hold your attention: a word, an image, your breath, or a visual cue.
Find a place to sit where you won't be disturbed for anywhere from 5 to 30 minutes. You don't have to sit cross-legged on the floor unless you want to. It's perfectly fine to sit in a straight-backed chair with your feet flat on the ground and your hands resting in your lap.
Close your eyes. Breathe easily and naturally, but focus on the feeling of your breath entering and leaving your body: notice how it feels cool as you breathe in and warm as you breathe out. As you relax, start to silently say a word as you exhale. It can be anything that's meaningful to you; many people repeat peaceful words such as love or simply one. You can also meditate on something visual: your child's face, a flower, the ocean, or anything that makes you feel happy and serene.
The idea here is to clear your head of your inner chatter. It's impossible to make your mind empty, of course, but if you find yourself dwelling on the subjects that stress you out, bring your attention back to your breath and the word you're saying or the image you're visualizing.
Meditating is most effective as a stress reliever when you do it regularly, so try to carve out a few minutes every day for your practice. I know that it's not always easy to meditate at first, which is why a great tool to help you get started is a guided-meditation CD. A couple you might find helpful are The Beginner's Guide to Meditation by Joan Z. Borysenko, Ph.D., and Meditations for Overcoming Life's Stresses and Strains by Bernie S. Siegel, M.D.
Start your meditation practice with this 30-day course.
Beauty bonus: Meditation causes stress hormones in your blood to drop and boosts your immune system.