By hand, usually while it's running. Looks like a sharpening stone. Different grits, sizes etc. They work awesome.
Remember if you are cutting MDF, it's essentially sawdust (and paper fibers) and glue. This glue gets very sticky when cut, so a pre filter (old sock, or two) works awesome (at prolonging the lifeof the router.
"Trust me, you're not my wife, so I have no reason to lie to you...."