There is no set formula to how I paint eyes. The essence of painting anything really is knowing your lightsource and the material you're trying to convey. With the eyeball as an example, the important thing is to first shade it like a regular sphere. In the end you apply the 'specular highlights/reflections", which is the shiny, mirrorlike quality that wet or smooth surfaces have. Sorry if that's hard to understand! Light is a subject that requires a lot of study.
I suggest getting photos of eyes (or better yet, entire portraits) and trying to paint them. Compare what you see in the photos to your own eyes in the mirror. Get a lamp and shine light from different directions on your eyes to see what happens. Good luck!
Oh my god.. O___O It's a magic!!11 When I have opened my page in here, I thought "Oh my... Stop, stop loading, I wanna get back and save it for myself!111!" Lol.. It made me mad. *o* Awesome girl. *__*