Paint it is about all you can do. If the plastic if thick enough, you may be able to sand down a layer and then polish the plastic. Plastic, especially the older polystyrene, did not have UV thermalset plasticizers that are added to keep UV and heat from causing the plastic to change in its molecular makeup. UV and heat will cause the plastic to not only yellow, but also will cause it to become brittle over time. Styrene is like your skin, it changes with time due to its exposure to various elements.
There are chemicals that are advertised to remove "yellowing". However, make sure to look at what type of plastic it is rated for. Most of these chemicals will only work on polycarbonates or ABS and will make styrenes even more fragile.
Scott