While it is hard prevent someone from crashing their car into your car, there are some things you can do to minimize whiplash injuries.
The first preventive measure is making sure your seat is upright and the head rest touches the top of your head. If there is a lot of distance between your head and the rest, it will do little to help in the event of a collision. Also, if the headrest is too low, in can act as fulcrum, leveraging your neck into a worse position, and increasing injury risk.
Another thing we can do minimize injury risk is being in the best possible shape prior to the trauma. Research has shown people with good aerobic fitness seem to be more resilient after whiplash accidents.
If you are aware you are about to be hit, it is probably best to remain in a neutral position with eyes facing forward. People with their head turned prior to impact seem to have worse ligament injuries.
If you can afford it, a larger car will lessen the momentum of your vehicle after a collision. There are now vehicles with whiplash-protection seats (e.g. Saab) that dampen the effects of rear end-accidents. Frontal collisions tend to be less severe to the neck if the speeds are equal because the chin can hit the chest preventing forward motion of the neck. Rear-end and side-impact collisions do not have this benefit.
Unfortunately, there are many simply unavoidable risk factors. Women and children seem to be more vulnerable, possibly because of the increased head to neck size ratio. Also if you’ve ever had a head or neck trauma before and or have a degenerated disk in the neck, these factors elevate your risk for a more pronounced injury. Other factors that can slow your recovery include wearing a neck brace/collar, taking to bed rest, or getting vertical traction treatments. Inhibiting movements can feel good initially but is not good in the long term because of decreased muscle function and strength. Getting diagnosed as soon as possible can determine the best course of action for getting you to back to a speedy recovery. Just letting things go is rarely a good solution.