They did a test of all these methods on one show (I forget the name of it). They all failed miserably. A clear shield will only produce flash glare if the flash is directly in line with your car, which it never is, so your license plate comes out crystal clear on the picture. They showed the results of the photos. Some methods did make it a bit tougher to see the numbers on the photos, but all they did was switch to negative view, and it was clear as day.
The only ones that work are the one's that are illegal - they're a polarizing type filter that obscure the view of your plate unless you're looking at it pretty much dead on.
It takes a big man to cry, but it takes a bigger man to laugh at that man.