We have a lot of threads and resources posted here about turning photos into paintings, but what about painting so that it looks like a photograph?

Obviously there's a big talent component here, but there must be some techniques we can use with Photoshop (or other editor) tools to brush on photo-realistic features.

This would be of benefit to our photo-based artists, of course, but also for our restorers. How many times have we seen critical information totally missing from an old photo, and our only option is to blend in something from another photo?

Blending only works sometimes. But sometimes you can't drop in parts of another image without essentially ruining the authenticity of the image. This is especially a problem with faces.

So, do you have any techniques or resources to recommend when all we have to work with are the editing tools to paint photographically?