Cloning; it doesn't always work out the first time.
If you see the error immediately you simply undo. If you don't see it until several steps down the road, the history palette is your only hope, but that can be a pain if it means reverting several steps.

Try this[list][*]make a duplicate of the original layer and use this layer for retouching[*]select the Rubber Stamp tool and select Use all layers[*]make the retouching layer active and start cloning[/list:u]
Any mistakes now can be easily corrected with the eraser tool