1. With a saw, cut vertical slits along the "spine", 1 or 2 cm apart, and just 2 or 3 mm deep (that's roughly 1/2" and 1/10" respectively, for the metric challenged).
2. Take some dental floss. Make a knot on one end, jam it in one of the end slits.
3. Run the dental floss alternatively up and down through the slits, in a S-like pattern. When you get to the end of the spine, run the thread again towards the other end, covering each slit again but in the opposite direction. After this, each slit has two bits of thread, and each cover and back segment between the slits is covered once. Secure the thread with another knot.
4. Apply carpenter's glue on the spine. Spread it so it covers the spine and thread fully.
This proves to be incredibly sturdy, and usable even in thick books, although for these it becomes difficult to keep open in the middle, as the spine is a bit stiff.