Meditation is an exercise for the mind and spirit, but the way you hold your body during meditation is an important foundation. In my articles on OOBE I talked about sending the body to sleep, while keeping the mind awake. When we meditate we are looking for a similar state. We want to stay awake, while being able to forget about the body. Lying down, especially when you are new to meditation, isn't a good idea because it's too easy to fall asleep. For the same reason kicking your feet up in a comfortable recliner is usually a bad idea. A sitting posture in which your spine is vertical usually works the best.
Patanjali's Yoga Sutras defines the meditative posture as "that which is firm and pleasant". This doesn't mean that the posture in which you meditate is comfortable in the beginning, often far from it, especially if you are older and haven't maintained the flexibility of your joints through some sort of stretching regimen. What it does mean is that over time the posture you chose allows you to lose awareness of the body while maintaining your posture. The lotus posture from yoga, in which the left foot rests on the right thigh sole up and the right foot rests on the left thigh sole up, is the classic example of this. For someone who isn't flexible, it can be extremely painful, if not impossible to achieve. Once you are able to get into the lotus, though, it locks the body into an upright position that will maintain steadiness while allowing you to forget about it.
Many people aren't able to sit in the lotus easily by the time they reach their teen years. For those of us who don't want to embark on a major flexibility regimen, or who are not able to do so for health reasons, their are several other postures that are almost as good and much easier to perfect.
The first of these is to simply sit in a straight-backed chair, feet flat on the floor about hip-width apart, hands palm down on your thighs. For people with arthrithis or knee problems this can be the best choice.
The second posture is the half-lotus. Sit on the floor. I prefer a padded floor or a thin cushion, which raises your buttocks and increases the stability of the posture. Place the heel of one foot against your groin. Place your opposite foot on you thigh, sole up. Either place our hands palm down on your knees; place your hands on top of one another palm up in your lap, or place your wrists on your knees with the palms facing each other. In the early days you'll feel some pulling in you knees and the ankle of the foot that is resting on your thigh.
The third posture is called the hero's pose. Here you kneel on the floor. Point your toes back and sit on your feet between your heels. If you are able, you can place your feet about 18" apart and sit on the floor between your heels. Put your hands palm down on your knees. In the early days you'll feel pulling on the front of the ankle until th ligaments and muscles there are stretched.
These three postures are easy to do, stable, and allow you to ignore your body as your meditation becomes more intense. They take practice, but the rewards in terms of being able to forget about your body and increase your depth of meditation are well worth the effort.