you can try novus plastic polish. I know some guys use wd40 on their quad to make it shine also. watch the graphics though it will make em soft. the wd40 also helps shed mud also. watch camo quads and make sure you don't do this or else you will end up with no camo.
Mothers plastic polish, Novus plastic polish. Mothers you can pick up at Auto Zone or Pep Boys. My Yamaha dealer sells plastic polish, but I forget what brand name it is. Plexiglass cleaner\scratch remover works too.