Meguiar's leather conditioner/protectant for seats/trim...works wonders. For the carpet floor mats, Tuff Stuff, Resolve, or soap and warm water.
The floor carpet is covered so it doesn't get dirty, but if it did I'd use the same stuff I use for floormats. In the winter I put black rubber mats over the floormats for protection. Then I Armor All the dash, door sills, pillars, etc so if something does get on them it will come off easily.
Then there's common sense stuff like not letting people WALK ON THE SEATS! lol I kid, but really no muddy/oily shoes, feet on the floor, no food and drink, no animals whatsoever, don't leave any trash in the car, etc.
My ivory interior, according to Josh Gilley, "looks like my car just rolled out of the factory".

So I must be doing something right.