Practice - practice - practice - practice
Earthgirl decided she wanted to draw better eyes back when she was maybe 5 years old. She drew eyes and drew eyes and drew...eyes. She studied eyes in other drawings, copied eyes from others' artwork, looked at little nuances. Last year, she sat me down to teach me to draw an eye, and we just drew ONE HUGE EYE on the page. It was the best eye I ever drew. I did not keep practicing, though, and now I'd have to start over learning to draw that eye. But my little Sm"artie can grab the sidewalk chalk and do this:
Practice makes perfect.