Getting the yellow out?

Does anybody know of a good way to clean up systems that have been
"yellowed" or am I just out of luck? I'm getting a Duo-R in the mail this
week that was listed as "slighty yellowed, as most this old are" and was
just wondering what I could do to clean it up (if possible). I'm pretty sure
it's just the natural process of aged plastic.. but I can hope for a better
answer, can't I? (:

-Leroy