I bought the Turtlewax headlight lens restorer kit for about $10 at my local parts store and basically followed the instructions using old fashion elbow grease for about 20 min on my driver side light. Got rid of the "fog" and it is just about as clear as my passenger headlight which is in perfect shape. Definitely a major improvement.
Check out this hyperlink for some additional details if you wish to do it yourself.
Best Headlight Restoration Kit Reviews