Polorized covers are somewhat effective.
At an angle, half the plate is obscured - straight on it's merely a cover.
Most cameras are mounted off at an angle beside the roadway, polorization may help.
I've found getting rid of the reflective ability of the white backgound to be very effective.
Either rough and scuff the paint around the letters, or paint over it with flat white.
The cameras rely on a badass flash to illuminate the plate background, while the letters don't reflect.
On a combination it's fairly effective, but I would not bet money on it.
I have a few other tricks that work on a truck, but are a little too obvious for a car.
This guy lives in Wickenburg, has a bit of a cottage industry going.
www.radarroy.com
Here's another site, seems to be unrelated.
www.radarbusters.com