First, take a look at the following partially complete images to see the progress. As you can see, I tend to fill in a section, and then move on to the next. The first photo below has a the blemishes on the eyebrow and bridge of the nose removed. NOTE: Once you get a part of the photo done to your liking - SAVE IT! Not only that, save it to a new file, so that if worse comes to worse, you can start over again with the original. I had four different files (other than the original) by the time I finished, with numerous saves to each file, so that each time I finished a major section I saved it to a new file. This way, if I decide later I don't like the way something looks, I don't have to go back to the start to get rid of it. Also, along the same lines, the Edit|Undo command is a valuable tool here. There were many times when I pasted in a part of the photo that was too light, too dark, or just wrong, and used the Undo command to get rid of what I had done.
The next image below has the majority of the face fixed. There is still a little more variation in her right cheek than I wanted, but I was not gaining much by trying to fix it more. There was a large area to paste in, given that I did not have much good area to paste from. I used several tools here. First, I used the readout tool to find similar areas of brightness to paste into the cheek from the forehead and from the area under the eyebrow. Remember, for that nice gradient of light to dark in the shadow of her cheek, you have to reproduce that nice smooth gradient. So, for areas where I really did not have the right combinations of shades to paste in, I used the Lighten and Darken tools (In Photoshop, this will be the the Dodge and Burn tools, respectively). These tools let you pick a circle of desired radius similar to the clone tool, and clicking on the area will lighten (or darken) the area in the circle. Using a reasonably large radius circle with high transparency (i.e. not much lightening or darkening) and high softness (blends in better with the background), I was able to lighten or darken cloned areas until they matched the surrounding areas of the image, which helped give that nice transition from highlight to shadow.
I was feeling pretty proud of myself for reconstructing the woman's face in the photo above, when my wife wandered by and said "what about that black spot by her left eye?" "What spot?" said I. "The big round black spot immediately next to her left eye (to your right in the photo above). "That's a curl of hair." "No, that's a spot." So, we got out the original photo (actually the photo of the original photo), and looked under a bright light with an 8x loupe, and sure enough, she was right, it was a black spot on the photo. Great. More face work. So I cloned in the edge of her face and a little of the hairline on the image below.
Not only did I fix the spot on the image above, but I worked on the background, too. This was harder than I figured at first glance at the original photo. Much of the damage is in that gradient from the dark gray background to the almost white surrounding area, and on the right side (yours) of the image, the damage was extensive enough it was difficult to replace without getting "stripes" of different shades. What I needed was a nice gradient with dark gray on the left, and white on the right, which I could paste into the image. Well, it does not have to be from the same image, so I created a new copy of the file, and flipped it to its mirror image. That way I could paste from the less damaged area on the left side of the photo to the more damaged area on the right side. This gave me the image above.
Finally, I cropped the image a bit, and made some minor modifications to the brightness and contrast, to get the final image below (right). Below left is the original image for comparison. Note that ordinarily I would sharpen an image before I finished (using the unsharp mask command - see Case 1), but not in this case. Just for laughs, I tried sharpening the image, and of course, it nicely brought out all the minor defects that I had not bothered with - it looked almost as bad as the original image.
A few random comments on cloning.
1) Start with an image you don't care much about. You'll want to practice before you get to an important image. It also helps to start with some minor damage and work up.
2) Saving here is similar to how voting used to be in Chicago - do it early and do it often. Do not save, however, until you are sure you like the result.
3) I find it helps to zoom way in to work on cloning, usually to the point that you can see individual pixels. However, zoom out often (or use the equivalent of the magnifier tool if you have it), because sometimes you don't know you are putting in weird patterns until you zoom out.
4) For large defects, try to clone in image parts from different areas and from different angles. This will help to avoid repetitive patterns.
5) Plan on spending some time and effort to do it right if there is major damage, especially if the damage is to something important in the image, such as a face. Conversely, if there is major damage to the image, and the photo is not important to me (or, really, not important to my wife, since it is her family), I just don't do the repair.
6) Play with the Darken and Lighten tools - they can help blend parts of the newly cloned image into the background if it is necessary to paste in something that is not an exact brightness match.
7) Despite the time and effort involved, the repair can be fun. It lets you exercise your artistic talent when you have to do major reconstructions.
I hope this helps get you started on using cloning to repair image damage. As always, please feel free to send me comments.