I haven't had anything clean MF as well as using a polishing pad rejuvenator like
http://www.autogeek.net/dp730.html
It's basically a very strong detergent with nothing that will interfere with the operation of MF (like most laundry detergents have).
Pre-soak in a bucket, and then you can also use the stuff as the detergent in a machine too.
Even an MF towel which seems clean will turn 3gal of water nasty colours (depending on what it was used for) when something like this is used.