This is my take on the tutorial that tellywacker requested. I will show you how to make your eyes look like the eyes in this photo: http://i38.tinypic.com/xmmidd.jpg I will be using my own photo for the tutorial and Photoshop 7.0. I believe you can do this with all versions of Photoshop, though.
Depending on how close up the eyes are in your picture, you may want to zoom in some. [The little magnifying glass in the Tools window].
Select the Dodge Tool: brush: soft round
Range: highlights
Exposure: 40%
With the Dodge Tool brush around the iris along all the lighter parts of the eye and wherever you want it to pop the most.