Another good tip is darkening the pupil of the eye. Humans make eye contact with about everything and the eye being in focus really goes a long way in a photo of a subject that has eyes. Darkening the pupil helps make the eye "appear" to be in focus (if it already isn't). Using the burn tool, or paint brush set on 'overlay' is good for that.

Like mentioned before though, do NOT go overboard, sometimes "less is more".