I just use car wash shampoo and a brush but it doesn't do a good job of removing stubborn stuff because it's ph-neutral. Other people use dishwashing liquid (e.g. dawn), or for stubborn stains step up to all purpose alkaline cleansers/degreasers like purple power, superclean, zep, etc but there are also some car-centric products like Meguiars heavy duty vinyl cleaner or mckees floor mat and cargo rejuvenator.
I came here to tell you about this exact product lol. I am quite pleased with it, used it on the cargo floor covering too. While it's getting time to use it again, it took years of wear off my mats.
two things I have done to clean floor mats,...in the winter I scrub them upside down on a clean icy snow bank. (this gets 90% of the mud off, leaving you with very little to work with ...or live with)
in summer: I walk in to a clean brook with them with a brush. and the water rinses the crap away. way better than a hose. (nice if you got "too much soap" )
two things I have done to clean floor mats,...in the winter I scrub them upside down on a clean icy snow bank. (this gets 90% of the mud off, leaving you with very little to work with ...or live with)
in summer: I walk in to a clean brook with them with a brush. and the water rinses the crap away. way better than a hose. (nice if you got "too much soap" )