Stretch,
Painting the eyes is always a challenge. In fact modeling figures is always one of the most difficult parts in building models. I know Paine Sheppard's building dioramas book has pages dedicated to the topic...His stuff is available through Kalmbach publications.
When you paint the eyes, I find it helpful to follow a few steps...
First, start with white and paint the almond shape areas where the eyes will go.
Next, take the color for the iris and paint that spot in the center of the white area depending on where the figure is looking. I find using a pinhead is useful for positioning the iris. Other techniques show painting the center with the eye color, then touching up around it with white. Either way works.
After you have the white and the eye color, take out your white and touch up around the eye color making sure the iris is in scale to your figure.
Finally, use your skin color to touch up around the white parts shaping the eyes as you want them to appear. Not too big nor too small.
A great brush as small as 00 might be necessary. I find working with acrylics is easier because the drying time is much shorter...however...for deep, rich color you might have to use oil or enamel based colors.
These techniques are all outlined in Shep's books and there was an article in finescale as well...If I come across an internet site I'll let you know.
the key is to be patient and not try to be perfect each time...work neatly and realize small touch-ups are necessary to make it 'perfect'...avoid working or touching-up until the area is completely dry to avoid paint build-up or caking up...this could ruin your finished result.
Good luck with that,
Tank