I use Photoshop often to resample photographs and perform minor edits, but I am confident I have never tapped more than 5% of its potential. However, I recently discovered several tutorials that may change that equation.
The most useful is Airbrush Tutorial I, a guide to the techniques used to touch up portraits. Not that the model above needs help, but the results are dramatic and not too artificial.
I also recommend Create Depth of Field and Professional Retouching. Each of the three tutorials offers clear, step-by-step instructions (complete with pictures) to guide you to the desired result.