Here are some examples of the before and after pictures:

Dodge Caravan - Very common for these models

Mitsubishi Eclipse

What causes this to happen?
The elements of the rain, sun, and being blasted by bugs, road dirt, sand, etc. cause the plastic to break down. Over time, they will become pitted, scratched, and take on a yellowing or a milky color.

What can be done to stop it?
Really nothing. I have another vehicle that has GLASS lenses, and they cannot be repaired using the methods that I use on the acrylic plastic lenses. They will have to be replaced. Unless I can come up with another way of reconditioning them also.

Will the problem reoccur?
Yes. If you keep the vehicle long enough and expose it to the elements mentioned above, they will eventually cloud up again. Generally speaking, this will take quite some time unless you live close to the beaches where the sand blows across the roads while you are driving thus "sand-blasting" your whole car! 

Do you guarantee your work?
Yes. I guarantee you will be satisfied, and if your vehicle was turned down during the state inspection process, I guarantee it will pass when the procedure is finished.