I usually do one of the three options below, depending upon how dirty the ceramic is:
1. Lightly dirty, after each sharpening session: Dish soap & water, scrubbing with sponge/paper towel.
2. More dirty: Comet powder & water, scrubbing with a green Scotch-Brite pad.
3. Very dirty: Bar Keepers Friend powder (similar to Comet/Ajax), mixed into a paste with water. Scrub with a green ScotchBrite pad, then rinse very thoroughly in running water. The Bar Keepers Friend uses oxalic acid, in addition to the mild abrasive. The acid dissolves the metal filings embedded in the ceramic. Very effective, but you need to use it with caution. The acid will damage some materials, so read & heed the warnings on the container.
There's also a product called 'Rust Eraser', which is an eraser-style block which can be used dry. Works pretty well on ceramics for quick scrub-downs as needed. I've also heard of people using pencil erasers in the same manner.