A 2 cm (3/4") thick, 45 cm by 75 cm (1.5'× 2.5') plywood board has approximately 1,000 16-penny nails pounded into it so that they extend through the board. The nails are spaced about 2 cm apart. A similar board, but with twice the thickness (actually, just two boards together) of wood, is used for the top of the sandwich. One or two thick books (preferably physics books) serve as a pillow under the head.
The forces (weight of the body, cinder block, etc.) are distributed over the total area of all the nail points so that the pressure is not high enough for the nails to puncture the skin. Of course we know what would happen if onewere to lie on just one nail! In this case, the pressure is extremely high because the area of just one nail point is quite small.
For more details on our setup see http://sciencedemonstrations.fas.harvard.edu/icb/icb.do?keyword=k16940&panel=icb.pagecontent193960%3Ar%241%3Fname%3Dindepth.html&pageid=icb.page80668&pageContentId=icb.pagecontent193960&state=maximizeSpecial thanks to Rob and Nils.