Here's a quick and easy to follow guide to painting eyes, along with a few tips that you might find helpful:
Never use a Pure White, use an off white or eggshell color, this also applies to teeth.
Individual lashes are not needed at smaller scales (1/4th to 1/10th), and rarely even at larger scales, with the exception of 1/1 scale.
Lashes don't grow around the entire eye, so they're not painted around the entire eye.
Dark Browns are better than Black when painting lashes. Lashes and eyebrows are rarely jet black, unless you're going for a makeup effect.
Always seal your work before painting the iris's, this will make correcting mistakes much easier.
Water colors are easier to correct than acrylics.
To make sure that eyes are facing the same direction, after painting the iris position of the first eye try painting the second with the model turned upside down. This will give you a better/different perspective, or alternatively cover one eye with your hand, and then cover the other once both eyes are painted, to make sure you're happy with the positioning.
Many thanks to Gentleman Jim Capone for this tutorial.