the yellowing of figures is a chemical reaction in the plastic itself. I can't explain it cause chemistry was one of my worst subjects, but my understanding is the yellowing is an indication of the plastic breaking down. you can slow the process by keeping the figures out of direct and consistent sunlight, not leaving dust on the figures and keeping them away from cigarette smoke.
there is a solvent out there called Goof Off that may be able to take some of the surface yellow out. I had minimal success with it and it probably does harm to the plastic, since it can be pretty harmful to people.