Retouching a photo is a complex task. Or not. It depends on many factors. Sometimes you need only a quick retouching because the original image is great and you just want only to enhance certain features or apply specific little retouches to it.
Colin Smith from PhotoshopCafe will show you in this tutorial one of the oldest trick for quick retouching in Photoshop. This trick is as old as blending modes and layers are!.
Colin also offers a free ebook at his web site with some interesting information about blend modes.