Work on avoiding chicken scratches, keep lines clean and smooth unless your art is supposed to be squiggly. If youre just going with basic contrast, Try to keep outlines darker than everything, or brighter than everything, like black or white. Mess around and boom

Do smooth lines, even if it takes a few tries until you get the perfect line, also if the line isn’t perfect you can always use the eraser to make the line more clean, also something I like to do is be sure all the lines connect each other, this helps at the moment of coloring and rendering