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.
bicycles.stackexchange.com/users/76/freiheit
You can avoid getting your pants dirty and save rotational weight at the same time by not wearing any pants at all.
bicycles.stackexchange.com/users/389/LanceH
You could get big socks and try tucking your pants into them.
I personally like these as they're waterproof too: http://www.sealskinz.com/socks
And for your shoes, you can get overshoes to protect them: http://en.wikipedia.org/wiki/Cycling_shoe#Overshoes
bicycles.stackexchange.com/users/472/Scott Langham
I use Helly Hansen Voss pants.
They are great
and I can take them off/put them on with my shoes on. http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=voss+pants EDIT:
Yes, I do sweat but not too bad as my commute is only 30 min.
If it's warm and raining I usually wear shorts and put pants in my bag.
bicycles.stackexchange.com/users/901/Andrew Boes
Like the other answers, I would recommend putting on special trousers for cycling.
For colder/wet weather, I use long cycling or hiking trousers (like these ); those have the additional advantage of protecting from cold an rain.
You can either put them on instead of your regular pants, then change at work (more comfortable, less wear on regular trousers, but awkward to change in public), or just put them over your regular trousers (easier to change, and warmer).
For warm weather, I just use common Lycra-type cycling shorts.
Yes, they are not very stylish, but they are very comfortable for riding, and changing into regular trousers is quite fast.
An additional option, particularly for short distances where changing trousers is too much hassle, are bike gaiters .
They are easier to pack and quicker to put on/take off than cycling trousers, but protect shoes and lower legs from dirt (and rain).
They are also a good alternative to rain-proof trousers, if you wear a poncho.
bicycles.stackexchange.com/users/1093/sleske
Much easier to wear cycling-specific clothing and just carry a change of clothes/shoes with you in a waterproof bag/pannier or leave some at work.
Anything more than fenders and flaps is going to be a hassle to deal with.
bicycles.stackexchange.com/users/1998/Angelo
I haven't cycled for years, but I walk about 9,5 kilometers to work every workday (and the same distance back), so I know the problem well.
From my experience the single most efficient measure is to wear shoes and pants of light colors - light gray, clay, etc, so that typical dirt is not that visible on them.
This fixes the problem once and for all
- I wash my sand color jeans once in maybe three weeks
and I just never wash light color shoes -
their outsoles get abrased to holes faster
(that is in about three months).
bicycles.stackexchange.com/users/628/sharptooth
I recommend Neos Overshoes, which both my wife and I wear.
You wear your regular shoes inside of these, and these "overshoes" add a little warmth, protection and traction, covering some of your pant leg as well as your shoes.
They aren't the most attractive things out of the street, but you may arrive perfectly dry and clean at your destination.
I have the Voyager NEOS , but they have a less expensive Villager model as well.
The nice thing about NEOS is their versatility.
They are useful any time you need to go out in wet/dirty conditions, and are not just for cycling.
The don't "clip in", but if that's important to you, you might consider a reversible pedal that is flat on one side and has a clip on the other.
I use such pedals on one of my bikes that I use for both and long trips.
bicycles.stackexchange.com/users/2001/Mark Stosberg
Gaiters such as skiers wear would be another option.
bicycles.stackexchange.com/users/1584/Daniel R Hicks
If it is wet, not necessarily pouring rain, it is easy to get the bottom of the pants and the shoes dirty. My tires have  protective plates fenders, yet it is not enough. If there a way to avoid that?
How to keep the pants from getting dirty
bicycles.stackexchange.com
<commuter><accessories><clothes><cleaning><weather>
bicycles.stackexchange.com/questions/2349
bicycles.stackexchange.com/users/882/Katharina Friedrich
There are a number of ways to protect pants and shoes when cycling. Protective fenders or mudflaps can be installed on the bike. The rider can wear protective pants and shoes over the regular pants and shoes or wear cycling clothing and change at the destination. Finally, gaiters are an option, particularly for short distances where changing is a hassle. A solution would be to use wider or better quality fenders, which would ideally cover the entire tire, or add a flexible mudflap at the front fender. It is also possible to use cycling-specific clothing, for instance protective pants, cycling shorts and gaiters. Moreover, you might want to put your trousers into bigger socks to protect them, or use overshoes that provide warmth, protection and traction, aside from keeping some of your trousers clean.
A solution would be to use wider or better quality fenders, which would ideally cover the entire tire. You could consider using a flexible mudflap at the front, in addition to the fender. It is possible to use cycling-specific clothing, for instance protective pants, like Helly Hansen Voss pants or cycling shorts. You might want to put your trousers into bigger socks to protect them. Gaiters can be used to protect your trousers, especially in short distances. Overshoes may provide warmth, protection and traction, aside from keeping some of your trousers clean.