One problem a lot of people have with sketching is that 'furry look' you get from really short lines that inevitably end up curved as you try to flesh out the shape of something. Instead, draw a longer line, with more movement in your wrist than your fingers, and try to, in one line, flesh out the shape of the line... you won't get it right the first time, so you draw again over that, with a quick movement, and then you'll still end up with that sketchy look as you see the overall lines you've drawn so far and, with each successive line drawn, create the shape you're looking for... This way, as you repeatedly sketch in lines, you'll get a better idea of what you're changing each time, instead of progressively adding small sketch lines onto the overall line you're going for... this might be a cumbersome read, sorry... the main point is, you sketch because you can't get the line right on the very first try... no one can. So it helps to draw the whole line multiple times over itself than to slowly create the lenght of the line, deciding along the way how it's going to change...
If that doesn't make sense, let me know...