I was recently asked in Noah's class by another student to share some of the secrets to my line drawings that can be seen from the previous post here Art Camp 2 Week 1. The big secret is patience, going slowly and experience, time. Here is what I told him though:
As for how I get my lines the way they are, and by this I am assuming you mean accurate or deciding which ones to pick out, the short answer is experience. I have been doing this since high school back in the early 90's. But for a better answer consider this when doing these kind of things:
Every shadow, highlight and in-between tone is a shape. It is as simple as that and if you outline each one, and I usually try to do it according to value - darker outlines for darker shapes lighter outlines lighter shapes, then you get what I did for the studies. To make them more accurate look at the study of N.C. Wyeth or the 2nd Bargue copy. There you can see a pretty tight grid over each drawing. They all have it but these are the 2 that I left it on. For copying photo reference there's simply no better way, except possibly tracing it, but I used to suck at tracing so I would just redraw everything (again lots of practice). At a small level and with patience it is easier to make sure you are getting the lines set right.
Now to do it from life there is probably no better way than the atelier way but it doesn't have to be size sight. I studied at the Safehouse in San Francisco for a little while which is where Karla Ortiz is from. She was a class ahead of me there. There we used plumb lines (anything that hangs straight on a line) and knitting needles to measure things by holding them out at arms length, taking a smaller measurement - say the width of the head because that fits into the height of the head and using that to lay in the outline of the form starting with straight lines boxing it in. So the head is about 1 1/2 head widths high. Easy enough. Now you know the basic size relation and box size of the head. From there start using smaller measurements with the more detail you put in and use the plumb line to check how things really line up vertically. You can also use the angles of the face like the triangle of the nose pretty much continues on to the corners of the mouth and to the start of the chin on each side. There is a good book on atelier drawing called "Lessons in Classical Drawing Essential Techniques From Inside the Atelier " by Juliette Aristides, check it out for a crash course on realistic drawing and how to measure out everything.
So basically define an edge for every "shape". Start with big measurements and the outside proportions. If those are right everything else can easily be found. If they are wrong your insides will be way off and so will the whole drawing. Then get into smaller and smaller shapes and lines within the drawing.
I did a quick walk through of the steps in another Frazetta study. I started with the black outline shapes. Then moved to the red reducing my grid by half then to the blue lines reducing that grid by half again to tighten up some details. It is still rough but basically what you will end up with is the final black line drawing. Then if you want to add tone just pick 3 or 4 tones from white to black and fill in the shapes accordingly. I go with 100% White, 100% Black, 50% Black and 20% Black for my tonal studies.