You didn't mention if you had an X-ray to rule out fracture or other bony injury. Assuming you have the usual soft tissue injury from a whiplash, here are a few tips.
There isn't one treatment known to work best for painful symptoms after a whiplash. Research supports an early return to regular activities. Pain relief can be obtained from the use of over-the-counter or prescribed pain relievers.
Ice is often advised in the first 36 to 48 hours. Cold may help keep the swelling down. Some experts suggest alternating cold and heat during the subacute phase from day three to day 14.
A soft-collar may be used during the first few days. The idea is to rest the head and neck muscles during the early healing phase. Longer use of a collar isn't usually recommended. Movement is much better to help with the healing process.