I like the phonebook idea. You might also take a look at the "Stropping, angle plus pressure" thread, as the exact same principles apply. You are trying to hit the edge at the right angle on a pliable surface.
ETA-Also with the phonebook, you have all that sharpness testing media right there at your fingertips...
ETA-Also with the phonebook, you have all that sharpness testing media right there at your fingertips...