Fooled with Grafix Duoshade paper a few times. It basically involves a pre-treated bristol board and two developing solutions. First draw your picture as you normally would, ink it and then brush on the first developer for a lightly lined effect. Now, brush on the second one for a darker area. It was a lot easier than laying in a bunch of Zipatone pieces to get the effect you wanted. Of course this was back in the days before computers (Damn kids, I remember back in the day...).
Never got as proficient as say, Al Williamson or the master Wally Wood but I enjoyed the process.