I've been using a meditation CD developed by immrama.org for about two weeks now and have been confronted with this fact: I have trouble staying awake when I meditate. You might think that laying down is a good position for meditating. I can last twenty minutes before I start to get the hypnogogic imagery that signals to me that I am slipping from deep relaxation into sleep. I even found a good way of keeping myself awake: I raise one hand so that my arm is perpendicular to the bed. When the hand begins to drop, it rouses me from slipping into slumber. The problem with this technique is that my mind is still foggy. So today I did some searching and found a few websites about posture for meditating (this is a good one).
Sitting is best for meditation. It doesn't have to be cross-legged or on the floor, but it does have to be comfortable. Often I feel knots in my back and this could be my effort to resist the weight of my arms pulling my trunk forward. To remedy this, I support my hands (palms up) with a pillow in my lap, using the classic meditation mudra, with the right hand resting on the left, thumb-tips lightly touching. This should help me stay awake, too, because apparently the thumbs drift apart when you start to fall asleep.