For the mats if they are really bad, when I pull them from the car, I scrub them really well with an OXY/Laundry soap mix. Then after letting them soak, I hang them vertically and rinse them from top to bottom with a heavy spray. When the water runs clear, repeat, let it drip-dry for a bit and then use the extractor.
For the interior carpets, there is a 'mist' setting on my garden hose nozzle. This works really well for pre-wetting the carpets and during the rinse. It's usually better to wet the carpest with water before using the cleaning solution - it rinses cleaner/easier that way. So it would be: dampen/cleaner/scrub/rinse/repeat rinse as needed.
If you're extracting a lot of dirt from specific areas after the water has mostly been pulled out, a second application of cleaner might be needed. Sometimes scrubbing during the rinse/extract - with a clean brush - can help.
You have to remember though, that on most daily-driven cars, there will be some dirt/stains that will not come out. You'll be able to fade the stains and improve the appearance significantly, but no matter how many times you run the extractor over the carpet, it will still pull up dirt...I use an extractor with a clear nozzle, so I can see the dirt coming out...even the tiniest bit. Sooner or later, you'll realize you've "hit the wall" and can't make any more of an improvement. The sooner you realize and accept that, the better off you'll be.
Of course you also have to realize that I'm talking out of my :ass in the last two sentences. I know all about hitting the wall...but I refuse to accept it (and I don't think many on this forum accept it either).