Here's an interesting version of what you can do with the eyes in an illustration. The client requested "more sparkle" in her eyes as her entire life people have said she smiled with her eyes. Here's the first version...



...and here's the sparkle version! They really pop. I created the difference by layering a series of the lighter shades within the eyes with a variety of gradients. Further I added additional shading and highlights to the outside of the eyes to draw in the viewer. Its a subtle but very powerful difference that really changes the viewers perception.