An extractor or dousing the carpet as described above will remove an amazing amount of sand, dirt and other filth. Canned carpet cleaners can't touch what water can do, and in the case of cloth seats, I prefer a strong wash to remove all the phart residue.
I'd add:
-make sure water doesn't overtop holes in the carpet, like seat mounting points. The carpet is rubber-backed and designed to get wet, but getting the padding underneath wet is not good. Car companies like to mount computers in stupid places, so keep an eye out, esp under seats.
- plan on having a plan to dry the carpet. Leave the windows at least cracked, run a fan, leave it in a garage, run the heater on high for a few hours, or any combination of the above will keep molds from sprouting.
- if there is staining that cleaning will not resolve, esp on a light color, then go get yourself a can of dye. Yeah, it's a sleazy dealer trick, but it works great and makes it look *new*