I've had good results from dripping dishwashing liquid (Dawn or Palmolive) directly onto the cerium and allowing it to sit for about an hour to penetrate. I follow that with a scrub with a toothbrush and BonAmi or Comet liquid under running cool water. Occasionally there will be a bit of hardened cerium oxide that's been in a crack or pit for decades that doesn't come out so easily, and for that let the dishwashing liquid soak, then loosen using an Xacto blade tip or needle before scrubbing. Some larger pieces with many polish-filled pits I've simply put into the dishwasher with the heated dry cycle turned off (you don't want to bake any stuff that remains, plus heat can put stress on fractures, veils, inclusions, etc.). A waterpik type tooth and gum cleaner works well in some cases (just fill the reservoir with warm water + dishwashing liquid) if you have one of those sitting around or can get one cheap at a garage/boot sale.