A very nice concept, but either the thinness of the paper, the strength of the scanner, or the lacking of the erasers has detracted from it... Perhaps retracing it onto another sheet would improve matters?
Does magic wand work? You can manipulate the contrast to get good lines to make the wand work better as well. Also, I recommend resizing the image down a bit more for people with lower res monitors.
post #18355 I used layers, gausian blur, and blending modes (screen), to extract the lines. Not quite there, but it'd be better if I had an image hasn't been hat hasn't been processed already. BTW, I'm not sure, but you might have applied the levels the wrong way. The purpose should be to lighten the smudges and dirt first, and THEN darken the actual content. And if it's impossible to lighten the smudges without destroying the content, then you need to go for advanced techniques, such as blur+layers+blending modes, perhaps even white/black spray to manually remove the harder parts.
And once you're done, scale the image down to about 50% size (pixel wise). It does wonders.