Proper posture in meditation is important. The foremost concern is that the posture helps us to be both alert and relaxed at the same time.
Posture can help support a condition of alert, awake awareness by keeping the spine straight and the neck erect. Human (and probably all mammal) physiology reacts to a straight spine by intensifying wakefulness.
Posture can help support a relaxed, open state by allowing the rest of the muscle groups in the body (besides those directly involved in keeping the spine erect) to release and let go as much as possible.
Furthermore, it is also important to be as comfortable as possible. Perhaps the most comfortable posture would be lying down, but often that does not support wakefulness.
A final issue is that the posture should feel stable and balanced. There should be no sense of instability, a tendency to tip over, or a feeling of being off balance.
The usual postures for meditation are:
- Crossed legged. There are many variations on this basic pose, but it is generally stable, relaxed, and the spine is erect. Many people sit on a pillow or cushion, although it is possible to directly on the ground. Allow your hands to rest on your things or in your lap.
- On a bench with the legs underneath. This is a variation on a kneeling posture, with the bench keeping the legs from falling asleep.Allow your hands to rest on your things or in your lap.
- On a chair. The most important thing to remember when sitting on a chair is to not lean against the back of the chair. Sit forward so that the spine must support itself. The feet should be touching the ground. Allow your hands to rest on your things or in your lap.
For numerous images of how to do each of these postures correctly, see the external links.
A large Buddhist site about meditation posture (this is extremely good!)
Seated meditation postures in yoga
PDF of an excellent booklet on meditation posture, called Posture-pedia