I second the bucket - type liners.
DH gifted me a set of Huskies when I had to buy a new used car. They pretty much earned their keep within a month, containing dirt, rocks, mud, and sand so it never made it onto the floor. Oh, and two full cups of coffee when my work thermos leaked :(. I just carefully picked up the liner and dumped it.
I'm planning to keep the car for 10 years or more. Giant coffee stains in the first month would have been a bummer.