I use an erasable non-photo blue pencil to lay in the basic shapes and capture the posture, proportions, and angles in most of my drawings. I then go over these lines with graphite and sometimes watercolor. Even though the pencil is erasable, I generally do not erase the lines. This pencil is so light and non distracting that it almost seems to magically disappear when you cover it with graphite. You could just draw lightly with graphite pencil for your starter lines but these lines end up showing much more than the non-photo blue guidelines.
While non-photo blue pencil strokes are easily seen on your paper (before you lay down the graphite), the marks are too light to be scanned and do not show up in my step-by-step tutorials. I usually approximate the effect of the pencils in Photoshop to create the instructional drawings you see on this blog or books. This is why the marks start off bold in the tutorials and then fade by the end of the drawing. On your real drawing, you will no longer notice the non-photo blue pencil lines once you put down your graphite over it (unless you reallllly look). I think this has something to do with the way our brains focus on contrast. Any neurobiologists out there please leave a comment if you know why.
Not all non-photo blue pencils are created equally. I use the Prismacolor Copy-Not Col-Erase non-photo blue pencil #20028. This makes the light ghost lines I need. If you use a regular Prismacolor non-photo blue pencil, it makes a bold blue line. I have also found that other brands of non-photo blue pencils make darker marks as well and I avoid them.
This pencil is very very light. If it does not make a mark that is dark enough to barely see, do not press harder, instead make multiple lines, circling back over the same stroke until it appears. If you press too hard and are doing a graphite pencil drawing with lots of blended subtle shading, the non photo blue lines will prevent some of the graphite from adhering to the paper, leaving light lines against the shading. Also on some types of watercolor paper, the non-photo blue pencil seems to act as a resist, preventing some of the paint from sticking in the same way. Slick paper may not have enough tooth for the non-photo blue pencil to catch and leave a mark. If you have these problems, you many want to go back to the light graphite pencil drawing to lay in your initial shape.
See a review of all non-photo blue pencils at JetPens (this pencil is thier top choice for wood pencils).