I only just saw this thread so i'll offer my contribution, although it's fairly comprehensively covered with the above tutorials.
I quite often spend time on the eyes and mouth to make sure they are well defined...if the model is looking at 'you', you should feel the 'engagement'...that's what i go for...and most photos, even hq ones, fall short in this regard. I use white to regain the shine in the eyes [often the photo has turned to a milky color]...and then i emphasize the colour of the iris, but try not to overdo it...i try to achieve a realistic look, and therefore subtlety and a 'painterly' eye is required. Lastly the hi-lite, or reflection, in the iris is applied.
The technique varies with how close the subject is...obviously you can do more with a close-up as with the Pocahontas morph, Nick referred to. A combination of pencil and paintbrush, and variation of hue and colour can give striking effects...also don't forget to work on the blackness of the pupil, and the definition of the eyelid with shadow underneath. All that helps regain the 3D impression and sense of 'immediacy'.
As i said, i also work on the mouth...often enlarging by 10-15%, reworking the form sometimes, and redefining hi-lites and shadows...i like a generous mouth with luscious lips...

As with all things, it just takes a litle time and patience...and practice to achieve what you want.
Some of what i've said is redundant, i know, but it may help to emphasize one or two things.