Learn the Rules Before You Break Them. Illustrator Abigail Larson says, "Do not forget that in order to break a rule, you first have to learn it! Do not underestimate the importance of rigorous practice in your subject..... If you have trouble drawing hands, take time to sketch at least ten hands in various positions every day and soon you will be able to draw them faster and with more accuracy.”
Line Weight is a Tool. According to illustrator Mike Corriero, "Line weight is a very important aspect of any sketch. I use a darker tone and thicker lines for forms that I want to stand out, while other areas are very light with loose lines. This helps distinguish what is important and provides a bit of depth and breakup to the drawing.”
Lines are Not Always Lines. Oil painter Sean Layh points out, “Lines do not exist in the real world, just the illusion of them. What you want to replicate within your image is the tonal contrast between objects. The lightness of an object contrasted against the dimness of another will simulate the line."
Make sure you read the complete article for more helpful tips.