I just put a little laundry soap in a sink filled with warm water and hand wash with a clean sponge. Takes about five minutes (or less). My mouse pad has a cloth cover, so using laundry soap on it is like washing clothes. Neither the water nor the soap have any effect on the rubber backing. Rinse thorougly and then air dry. I do this maybe two or three times a year. Works perfectly.
I've also been known to just buy a new one if I don't feel like taking the time to wash the pad. I use the really large pads made for draftsmen and graphic designers -- Super Mat, I think they're called. They cost all of $7 or $8 and last for a couple years if cleaned semi-regularly. Great product.