Usually hard plastic is hard to get scratches out of....no pun intended. Have you heard of a product called "Back to Black"? It comes in a small, red bottle at most automotive stores. If the scratches are light colored in nature, Back to Black will definitely help remove the scratches.
If the scratches are actually cut into the plastic, then your best bet is probably to just replace them, but let's see if anyone else has better suggestions here before doing that.