Why does this make me think someone is going to be looking for a bucket of prop wash?
To the question, if you didn't go with the cover that killr suggested and the plate is dirty, car wash like on the rest of the car usually works. If the plate is painted, you can usually treat it just like the paint on the car, as far as APC, tar remover, polish, etc. Just be careful a lot of the newer plates use some sort of vinyl applique (usually for reflectivity or to add a watermark) and you can damage that more easily, like if you're buffing your friend's license plate with your PC because you did your painted one the same way...don't ask me how I know...