Below is a picture of the engine bay of my Leon after six years of ownership, pretty much as it was when it came out of the factory.
All I ever did with it was wipe it down with a sponge and the water/shampoo left over from washing the car. I think the trick is not to let it get too dirty in the first place.
By way of comparison, here is a picture of the engine bay when the car was brand new.