I've had good luck with "Flitz" metal/plastic polish reducing mild yellowing, but you have to be very gentle as the polish can change the surface texture if you get too aggressive. (Especially with A1200, A600, and later 1084 cases. The earlier stuff is much tougher - for a test, I hit a *badly* yellowed A500 mouse, after some fairly heavy rubbing it looked almost new, and the surface texture was still 100% intact.)
For anything more substantial, plastic paint (dye type) is the best idea IMO, as it penetrates into the plastic without leaving a coating... no dust inclusions or runs to worry about.
Also, I use Plexus for the occasional wipe-down, it keeps the equipment looking like new and protects from further yellowing.