What exercises can I do to improve the line quality of my drawings?
Sitting down with a piece of paper, and drawing continual lines. Over, and over and over again. Filling up a whole paper with just lines. Sometimes I will sit down and just make loop shapes on pieces of paper, to exercise the fingers, or zig-zag patterns, for short consistent lines. Practicing cursive handwriting has helped with line control in a similar manner, and able to get the flow of motion in the fingers, wrist and arm. 

How to create lines, you are using different parts of the arm.  

Drawing a line might seem easy, but determining what part of the arm to use can create different lines.
Line a) which is significantly longer line. Would require the muscles in the elbow.
Line b) which is half the size of line a. Would require the wrist.
Line c) Which is short simple lines. Would be using the fingers.


Elbow and wrist lines would be focused more on contour, while fingers would be more for hatching(shading with closely drawn parallel lines), or fine details. Having control of the arm, wrist and fingers. Will help with practicing drawing lines.