Most members are trying heat set paints on their polymer clay sculpts. Using these paints is similar to using china paint as you 'set' the paint between layers. Layering is very effective when painting the eyes.
This photo tutorial (by Patricia Rose) shows some techniques when painting eyes using heat set paints. http://www.patriciarosestudio.com/html/tut_1c12.html Try her techniques and see how they work for you.