The most essential point of the meditation posture is to keep the back straight, like “an arrow” or “a pile of golden coins.” The “inner energy,” or prana , will then flow easily through the subtle channels of the body, and your mind will find its true state of rest. Don’t force anything.
The lower part of the spine has a natural curve; it should be relaxed but upright. Your head should be balanced comfortably on your neck. It is your shoulders and the upper part of your torso that carry the strength and grace of the posture, and they should be held in strong poise, but without any tension.
Sit with your legs crossed. You do not have to sit in the “full-lotus” posture, which is emphasized more in advanced yoga practice. The crossed legs express the unity of life and death, good and bad, skillful means and wisdom, masculine and feminine principles, samsara and nirvana, and the humor of nonduality. Rest your hands comfortably covering your knees. This is called the “mind in comfort and ease” posture. If you prefer to sit on a chair, keep your legs relaxed, and be sure always to keep your back straight.
Source “A Glimpse of the Day” – http://usa.rigpa.org/