One of the weakest areas of my drawing has always been drawing eyes. I've never been entirely sure of the right way to proportion them out.
The suggested way to draw was to think of the eye in a couple of different of ways. First, keeping in mind that the eye travels back into the head as a sphere. This is common-sense when thinking in realistic terms, but it would seem that maintaining certain realistic elements of anatomy seems to help tie the features into being a lot more believable .
Secondly, continuing the idea from the last point, dividing the eye into three sides: upper lid; lower lid and edge of the eyeball.
The last two pictures above were each experimenting with two different types of eyes (hence the slightly crazed look to each). I found that after a little practice, they eyes started to look a lot more believable. Numbering them, left to right, 1 to 4, eyes 1 and 3 feel the best. Initially eye number 2 was intended to be a duplicate of 1, with an opposing slant one side. Eye number 4 started out the same as number 3, however I found it hard to match curves on the outer corners.
So the main challenge going forward is practising reflecting the curves to both sides of the face.