1. Zoom into a damaged area
2. Click on the "Stamp" icon to use the Clone tool. Hold the Alt key and click to select a region. Click again, and that region will be duplicated.
3. Use a soft brush with a small size. Reselect a cloning region periodically to make sure the clone isn't too obvious
4. Use a smaller brush for more detailed repairs
5. For complicated textures such as skin, use a lower opacity brush
6. Use Image > Adjustments > Layers to repair overall lighting