A box of 3M red Scotch-Brite pads, P.N. 7447, should be in everyones garage. You could use some of these and a medium strength degreaser like Simple Green and get good results using a bit of elbow grease. The nice thing about artificial steel wool is that the the used pieces coming off the pad do not rust. I like using real steel wool also but there is a place in the garage for both.
There was an article in Dirt Rider magazine I think that advised to clean/scour the glaze that builds up on the rotors every so often.