Cleaning mouthpieces and removing discoloration

How do you clean a mouthpiece?
DON'T soak a hard rubber mouthpiece in hot water. It makes them smell like rubber and changes the color.
Soak the mouthpiece in a solution of cool dish soapy water. Then gently clean with a tooth brush, and swab out with a handkerchief.
If there are calcium deposits inside, add white vinegar to the soaking mixture.

What do you do for hard rubber mouthpieces that turn drab olive in color? Here's the answer from Tom Ridenour:
Get some good black shoe/leather stain (not shoe polish). Apply it, let it dry and then rub it down.
The rubber does not change colour. The black dye leaches out; natural rubber is white/ivory coloured.

NOTE: I have not had good success with Tom's method. Professional repair shops have buffing wheels and black buffing compound that seems to work better. Soaking a mouthpiece in bleach has also been suggested, and seems to help a little bit. I would like to hear of other methods.

More recently I have had some success for a 3M product for restoring the lustre to plastic car headlight covers. It is 3M Plastic Cleaner and Plastic Polish.

Something new has been found!