I use the GIMP but it works in PS at least. Use the Dodge/Burn tool to lighten the iris of the eye just a bit. It looks most natural if you only do about 1/3 of the iris opposite of the highlight. Also, don't just lighten the whole thing, do it in rays from the pupil to the darker ring on the outside of the iris.
Here is a tutorial on what I am talking about. I think that the example is overdone, as it uses the airbrush instead of the dodge/burn tool. IMHO, dodge/burn is much more subtile if used correctly.
Peachpit: Portrait Retouching with Photoshop 7 > Accentuating the Eyes