To my understanding, the various physical postures for meditation and, indeed, prayer are related to harmonizing the energy flow throughout the body.

So, for example, the Christian method of praying by putting the palms of the hands together basically creates an energy loop through the joined hands which promotes a more relaxed mental state.

Regarding meditation with an upright spine etc., my own view is that it also prevents you falling asleep :)

I've tried meditating in bed at night a number of times and have always failed to stay awake. Sitting in a comfortable chair, however, keeps me awake without any problem.

From my various studies of the subject of meditation over the years, I have come to a very simple and straightforward approach...whatever is comfortable for you is right for you.

Sometimes that idea can lead to you finding something quite unusual and unexpected as your preferred posture...on occasion, I actually find it quite beneficial to meditate standing up in this stance...

Agreeing with Stingray, I will just add in that attempting to meditate whilst lying do**wn, inevitably for me, **makes me fall asleep (regardless of the time of day).

Yes, do whatever is comfortable for you. Personally I sometimes sit in a chair with my legs crossed over and my arms folded. I found that switching to sitting straight up in the chair and the hands in a neutral position boost my energy and concentration. Follow up on some Alexander technique for this.

well their is posture to make excersize like hatha yoga. but if you meditate for the soul or the spirit i would say that the best posture is one that you are confortable enuff in it so that it does not bother you or stop your focus. so experience and enjoy.

update:

also once you master soul and lose the sense. the body will not disturbe you no more then you can move to spirit level.

experience and enjoy.