First, make sure your headrest is in the proper position so that it protects the back of your head. Second, don't recline the seat too far back -- in a rear-end collision your whole body could actually slide back and forth and your spine contracts and expands like an accordian. You might also consider installing a flashing third brake light to help avoid rear-enders altogether.
The 'whiplash' test is new this year. If you look back I don't think ANY vehicle that wasn't made new this year and tailored for the test get a good on that test. The Prius is fine in that area compared to all it's chronological siblings. I'm sure they'll keep upping the ante on tests as they have been doing all along. Once everyone starts getting 'goods' it's time to go to a different test so at least some people 'fail'
