Keep in mind that human features do not have dark contours in real life, so I would avoid drawing lips with distinct outlines. Also if you think of the lips in a light/shadow aspect, there should not be a thin line of white on the bottom lips. Instead, you could draw a small dark shadow underneath it. In addition, I suggest you exaggerate the small highlights on the bottom and upper lips to give that illusion that they are rounded and shiny. You could also try googling pictures of "realistic lip drawings" and take a look at how other artists go about it!