How to keep the pants from getting dirty
Protective plates?  You mean Fenders/Mudguards?

I can think of 3 things that could help, other than simply bringing a change:


Bigger fenders (wider, and/or go down the tire more).  Basically, some fenders are better than others. A front fender that's wider than the tire with a stay (rod) going back for support that hugs the tire fairly closely  and extends down to the height of the bottom bracket is ideal.
A mudflap hanging from the bottom of the front fender.  This is a flap made of something flexible (rubber, leather, etc) that hangs from the bottom of the fender.  It effectively extends the fender down even further without it being a problem if it hits something.  Some fenders/mudguards come with this. Or you can add one.
Protective pants over your regular pants.  I have a pair of yellow rain pants that I put on over my regular pants when cycling in the rain.


Nothing will be 100%. It's always possible to get splashed by something else from the side.