Clean rag or two
Toothpaste (any kind should work)
Not pictured - bowl of water
Step 1: Apply some of the toothpaste on the dry cloth
Step 2: Rub the toothpaste on the headlight in a circular motion until the grime starts to come off. This takes a bit of elbow grease and may take a few applications.
Step 3: After it appears that the grime has come off, use the cloth (or another clean one) to rinse the headlight clean. If it seems that it is still a bit hazy, do another application with the toothpaste and rinse again.
Step 4: When it is completely clean, buff it dry. Admire your nice shiny headlight!
http://crazydazedesigns.blogspot.ch/2012/03/budget-diy-cleaning-foggy-headlights.html