The best way to improve your lineart is study references, duplicate your references as best as you can, and then do it again. One of my biggest epiphanies in pixel art (Well, this holds true to any 2d art venue) come with the understanding that I was trying to draw 2d objects and shapes, and then making it look 3d. In fact you should be doing the opposite. Before drawing anything, visual the 3 dimensional object, then translate it to a 2d depiction.

Take a look at this picture. Make note of the shape of the forehead, the protruding cheek bones, and the many other various curves. One way I like to practice shapes I don't much experience with is to block out shapes in layers. With a skull I would block out rear dome first, then add each subsequent layer over the last with a lighter color. Mid section, forehead and jaw, cheekbones, etc., saving the furthest point for last. This will give you a decent sense of depth on the image, and a better starting point for adding the arms.