This video by Aaron Blaise is a really great showcase of how to use photographic references to create realistic textures in digital paintings. This tutorial definitely leans more in the illustration direction than most photographers go, but there’s a lot of really great tips and tricks in there that could very easily be applied to photographic retouching. The trick Aaron does at 9:35 using an erodible brush to emphasize highlights on the wrinkled texture… I could definitely see myself using that one on an image in which I was trying to bring the viewer’s attention to something like an elderly man’s skin texture.
Looking outside of straight up photography for ideas and techniques can be very very rewarding. Suddenly your influences and teachers become endless.