Clean plastics with a soft cloth and mild detergent in water... NEVER use anything else!.
The yellowing im afraid is a reaction of the plastic with natural UV.. theres nothing you can do about it.. all clear plastic products have some sort of reaction to UV light be it yellowing or going whitish and cracking.
Clear plastic is very susceptable to corruption with other products... alcohol based products will turn them a bluey misty colour which cannot be renoved... once done its permanent, so mild detergent and water only and ALWAYS dry with the print on the upside to prevent it sticking to anything underneath ( very easy to lose artwork like this)... same goes for ramps.. steer clear of any alcohol based products!