Tips for Skin Retouching

Tip 1:
Lear how and why make up is used! It’s really important to understand makeup if you're going to be a good retoucher. It can also help you become a makeup artist.

Tip 2:
Pore Structure: Usually you don't want to sand blast someone's face. Real people have pores and you want to keep them as much as possible. In the highlights you may want to tone down pores that are very high contrast and prominent, but in other areas it's not as essential and can even create a plastic look.

Tip 3: 
Not all wrinkles are created equal and some are actually good. They make you look human. No sand blasting, remember? So forehead lines, laugh lines, and other significant wrinkles should remain. You may want to reduce them somewhat (remember it's just a darker shade next to a lighter shade), but you want to keep them. Good retouching is about taking a few years off of someone, not genetically re-engineering them. People are not pefect and attempting to make them so usually doesn't work out well.
Thinner, smaller wrinkles are usually fine to get rid of, especially the ones underneath the eye. Although again you want to be careful about making it look natural. More significant wrinkles can be made shorter and less 'deep' by dodging and burning.
 
Tip 4:
Know where your light is coming from! Where was the light source when the photo was taken? You can usually figure this out by looking at the catchlight in the eyes or the specular highlights on the cheeks.
When retouching you may want to create additional depth to the image, for example, burning in additional shadows. To do so, you need to identify the natural contours of the face to accenuate them. You can't do this correctly unless you know where the light is coming from.

Also, if you add makeup digitally, like eye shadow, you'll need to adjust the color to lighten the eye shadow where you would expect the highlight to fall. You'll use one shade for the eye shadow, but a lighter shade dabbed on top where you think a highlight should be.