Here's a pile of miscellaneous information that might help you draw manga-style eyes.

The Long and Short of It

Notice how the upper lid of the eye is almost always longer than the lower lid.

In-creasing Anxiety

When the eyes are opened wide, a crease forms above the upper lid. It makes the face look alert, and when you leave white around the iris (as in this picture) it gives the chara a surprised expression (don't forget to raise the eyebrows in that case).

Bright Eyes

To make eyes look brighter or lighter, try erasing or not connecting the line in the lower iris (see arrow).

Flat or Curvaceous?

The lower lid can contribute as much to a character's appearance as to their personality. A flat lower lid will make large-eyed chara look child-like, but it will make narrow-eyed chara look even more evil. A curved lower lid will, as usual, make the eye look bigger.