In this ongoing view of the Illustrator tutorial, I am recommending http://vector.tutsplus.com/tutorials/illustration/how-to-create-a-tube-of-paint-and-a-brush-using-adobe-illustrator/. If you learn these skills, you will turn flat graphics into ones that show depth, perspective. Also, when is the last time you have used penumbras in designing illustrations? If you do this tutorial, I bet you will soon become a better designer, because your eyes will be trained to look at light, shadows, blends, and so on. Let me know what you think? How many of you already think of these techniques when creating illustrations?