I think this is one of the most under practiced things when people sell cars that makes a HUGE difference to an interior. So many times when I've been looking at car ad's do you see mucky footwells or tired/ripped/manky carpets. Makes an entire interior look drab!
Heres the difference on my last 202 interior just by adding new carpets (And polishing lol):
I used 1001 carpet cleaner on mine and they came back like new. I left the stuff to soak for a couple of hours before gently scrubbing with a nail brush.
Also cleaned my seat belts in a hot water / 1001 mix. Left them soaking for a few hours and again, like new!
Totally agree with Mercfanuk, i hate it when you get in a car and the floor is mucky. I cant explain why but it always makes the rest of the car feel grubby and damp.