I feel pretty fortunate, none of my C= gear has yellowed yet (knocks on wood).
All I've done is keep them out of direct sunlight, and only clean with a little water, no chemicals. And of course, I never smoked around them...
Also, I live in a very dry climate, dunno if that helps any.
Throwing the casing in the dishwasher DOES help sometimes, especially if the yellowing is from smoke. Rubbing alcohol may also be worth a shot.
cheers
-d
