The other option is to lay on your back, with your legs propped up and your laptop balanced against your thighs. While putting little stress on your back, this can be hard on your wrists and your neck. To make this more ergonomic you need some kind of laptop stand to raise up the keyboard to a more ergonomic angle and move the screen up as well, so you don't have to bend your neck as far forward to see it.
If you don't need the laptop to be raised very much (ie your neck is still very flexible) then a fairly effective stand can be fashioned from a sheet of cardboard. I'll put up a post later showing how to make one. In the mean time, I'll be considering commercial options which give you a lot more control, one per post (click here for a list).