Mostly you just need to work on the shape of the face's outline and the interior shading. There's little bows under the eyes that are darker, shadows along the sides of the nose, shadows under the eyebrow around the eye (the eyes you drew look a lot like the renders of lenne, which were really pathetic. Lenne's eyes scare me really badly because they're so inhuman xD)

Eyes: You wont' usually see whites under the iris when a person is looking straight at you, and if it is, not very much. take tastetherainbow's sig for an excellent example of this xD Most people also usually forget to add the eyelid, either in proper thickness or all together. My friend is in art I and they're doing portraits, so I have to keep telling her that her faces' eyes have no eyelids on them xD it just wraps over, about 1/5 of the height of the eyeball. there's another one on the bottom that's smaller. Eyelashes follow the contour of the eyelids.
Low down: they're way too big, and you need to add an eyelid over the top and bottom.

Face Contour: The forehead is typically about two inches tall, it slopes gently out to the left eye, then darts out more drastically where the eyebrow is. it dips in further than it had been where the eye is, then goes back out to the eyebrow's area, slopes down over the cheekbones, if there's baby fat down around the mouth, it'll pudge out a bit, then come back in and be draped over the chin rather drastically again. the chin on the right side continues nearly straight back (you have to take in the effect of perspective though, so really it goes up a bit) then turns up right before you get to the ear. Of course you can't see her ear in this picture, but if you take her right eye, the tops of the eye and ear are on the same parallel. and the bottom of the nose and bottom of the ear also align.
Low down: make it curve in where the (smaller) eyes are, and don't make it go straight up after you get to the chin.

Shading: eh, just get a mirror and have fun, k? xD
Low down: darker around the eyes on top and bottom, along the sides of the nose, around the shadowed edge of teh chin; from the chin tot he ear is a dark area over to the lips and up to the cheekbones.

Hair: use some more strand definition, at least where the roots are, do more highlights that wrap around the sides of the head (not just lighter in the middle of each lock, highlights should curve like an oval around the head), and add some frizz xD there's only like about two people you'll ever meet irl who can keep their hair perfect constantlyx D

phewxD I hope that wasn't overkillxD but it feels like I have drawn a million portraits in the last three years of art class, and I don't want it to go to waste