It's just an issue of how long you make the eye lids, usually either the top eye lid is longer than the bottom, or the longest part of the eye will be in the middle between the two. Let me dig up some references.
http://i.ytimg.com/vi/Hd88gyw11lc/hqdefault.jpg very awesome realistic drawn eye.
http://www.absoluteanime.com/naruto/shino-eyes.png might as well reference a popular anime, notice the guy's eyes has the top eyelid longer and the side angles down into the bottom (the glasses kind of hide it but you can see the sharp inward angle it goes at) While the girl's eye is more equal length on top and bottom. the top of the eye does curve down slightly before it ends, but you can see where the top of the eye ends and the bottom ends is equal. (the bottom of her eye also angles up slightly)
http://tinypic.com/view.php?pic=343hkir&s=5 last example, if you prefer bleach instead, here's an example of another female and male (although for him the speech bubble kinda cuts off the eye)
I hope this helps clear up a few things. Also if you want examples from your own heads, head 1 that i did the eye for and your head six have the best eye shapes.