Try sitting straight up, or only semi-reclined, instead of laying down. I have the same problem. I can fall asleep anywhere, so when I would try this, I'd do the same.
If trying by sitting straight up, make your back as vertical as possible, and find your balanced "sweet spot" so you exert very little energy keeping straight up. Let the rest of your body: arms, legs, back muscles, everything, fall into total relaxation.