Well, it depends.
Keep your toys out of direct sunlight. When you are storing them, be sure it's in a dark place. I use a black plastic storage bin.
If you smoke, wash your hands before handling the plastic. And DO NOT smoke anywhere around your toys.
Take these steps, and it should help prevent oxidation. Keep in mind, there are thousands of different plastics used throughout the years. Different plastics will oxidize for different reasons. But if you follow the tips above, you will defiantly increase the lifespan of your toys.
Once the oxidation has started, there's nothing you can do to "fix" it. But, if you follow the steps above, it usually won't turn any more yellow than it is now. Usually, but not always. Again, different plastics...
Hope this helps a bit. Sorry this happened to your fighter.