I use the ink tool in gimp for my lineart, I find photoshop brushes to blurry and gimp is nice and firm. (I think I read use use photoshop). Try to use a single line, make it smooth - use your arm, not your wrist. Try to make your lines different strengths and widths, thicker in the darker parts, to give it a slightly more 3D look. Put a lot of detail in the lines, making sure the ends are all neat, and zoom in and out a lot.
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Not that I do any of that, that's just what I tend to find in a lot of tutorials. Just practice a lot.
Practice improves your line work. Work zoomed in at high resolution. Work at at 2 or 4 times the finished size and then scale it down before saving for web. Make brusk strokes with your whole arm and not just your wrist. Make brisk confident fluid strokes instead of slow hesitant wobbly ones. Or use line smoothing or vector if available.
Work on the lineart with a small pen/brush, really close, at a high resolution. It'll look really neat when you're zoomed out. Also, the stabiliser tool in SAI is amazing