I used Meguire's Heavy Duty Headlight Restore Kit.
Some have even taken the car to Sam's Club, they'll do it for about 20 bucks.
Good idea to put on Expel or Laminix clear plastic covers to protect them once you get them pristine again.

( A NEW set of headlights will run you around $1000, if you go that route, I'll gladly adopt your old ones as a charity )