There's many ways to meditate- this is one I think is quite lovely and peaceful.
Although most meditations you do sitting up so your spine is straight, this one you do lying down.
Lie flat. Keep your eyes closed.
When you breathe in, visualize a great light entering through your head into your body, as if a sun has just risen close to your head. Imagine the light is pouring into your head and going all the way down to your toes.
When you breathe in, continue to imagine the light pouring in and cleansing your whole body.
When you breath out, visualize a dark river entering your toes and going out through your head.
Go slowly and breathe in and out through your nose. Inhaling to the count of 6 and exhaling to the count of six is a good starting point... then increase over time as your lungs allow. Again, do both the inhale and exhale through the nose so you cool your system.
I find it very relaxing. And a nice way to take time for ones self... even if just 15-20 minutes a couple times a week.
(Note I learned this meditation technique from "The Everyday Meditator" by Osho- http://www.amazon.com/Everyday-Meditator-Practical-Guide/dp/0804819769/ref=sr_1_1?ie=UTF8&s=books&qid=1236096118&sr=8-1)