This is a eye drawing exercise I have done. I did both the side view and the front view of the eye and I find that the contour of the eye is especially hard to master. The most important part to capture is the different faces of the eye lids for the front view of the eye. The side view is pretty easy to draw, except that you have to pay special attention to the highlight of the cornea which makes the eye looks translucent if done correctly.
Eye Side View
Eye Front View