Lemon juice and salt are good for cleaning. But, darkening areas are more likely patina (an actual change in the composition of the metal, on the surface - which, if you remove, you are also taking off an exterior skin, like polishing jewelry), caused by years of sweat or moisture, salt in the skin, air, etc. If you were to treat the entire piece for patina, you are going to lose the years of character and possibly the associated provenance of an older garment. I would personally recommend leaving as is. Since the lurex and other more modern metallic fabrics have been made, it's not difficult to find a shiny one.