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. You can see in the photo that the light is coming from the side, almost directly so. The eyes are usually going to reflect where the light is, creating a catchlight.
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.
Contouring and digital makeup are going to be extremely difficult for video, so this is more of a photoshop/photography tip. Contouring can sometimes be achieved using the Curves filter in After Effects, so it's not impossible. But adding makeup to video is a big budget thing.
Bonus Tip #1:
Use a Wacom tablet for retouching in Photoshop. If you're going to add digital makeup or use the Photoshop tools, a tablet will make your life incredibly easier and allow you to do better work. There are other tablets out there, but Wacom is the best by leaps and bounds.
Bonus Tip #2:
Our Beauty Box plugin is a great way to do this type of work for Video (and it's being released for Photoshop in a few weeks). However, no tool is pefect 100% of the time. Even when Beauty Box doesn't completely do the job it can get you most of the way there, leaving only minor adjustments to be made.
And remember... There's nothing as humbling as retouching a photo of yourself. :-)