I like to use a multipurpose cleaner that I buy at costco cut about 32/1. Meguiars all purpose cleaner plus in the detailer line is nice too. I spray it into the mats, allow it to soak for a minute or two, then wipe with a towel, flpping to the clean side for final wipe. The best way is to extract, but that can be a really expensive piece of equipment. The little green clean machine by bissel works well and you can find them on sale for @ $40.
Folex is a great carpet cleaner that you can usually find at target or in gallons at some home depots.
I had crusted on week-old mud on mine...get a carpet cleaner 'soap'--don't use dawn or dishwasher concentrate cause it suds up too too much. But since the difficult thing is rinsing out all the soap really good, I used a super high pressure setting from a hose attachment (like the high stream setting). Take a towel & step on it to absorb most of the water, then let it air dry. It may take a couple days, but I would be concerned about it disfiguring if you put it in the dryer (esp. not the washer machine)