Back when I started drawing women I wished for long flowing lines to match the ideas in my head.
I found that the vellum paper finish (tracing, onion paper) against the wax of the Col-erase pencil made my lines smoother with minimum smudging. The strokes seemed to flow with almost effortlessly. I stopped using vellum but I do believe it helped a lot.
If you're looking to achieve more fluid and cleaner lines, give tracing paper a try.
F.U. After you have done the preliminary sketch, lay another sheet of vellum on top and go over it with the pencil as if you were inking it. Only this time you draw faster and without fear, using longer (and less) strokes. I used to do this several times until the drawing became sharper and streamlined.
By the third or fourth re-draw, you will clearly see the difference between the first and last sketches.
For really long lines, do what architects and industrial designers do when they project, put a dot on each end and draw the line in one take from dot to dot.
The idea is to gain confidence in your stroke, not much different than playing baseball or throwing a football around, the more confidence the more chances you'll take.
By drawing on top of your own sketch you won't be ruining the original so you can be as bold as you can be.