Periodically while I work on a portrait I use Photoshop to overlay a photo of the painting on the source image. Not a fancy technique by any means, but a hugely useful one. By overlaying the images I can see immediately where my painting is off. For example, in the first series below, I can easily see that my painting is too straight on. Rather than directly facing the camera as I've painted him, the subject is turning slightly toward his right. In the second series I see several issues. The most important of which are the tilt of the head and the placement of the right eye.
Capturing the spirit of the subject can't be helped by technology. Thank goodness at least likeness can.