3 Answers

When I was in the restaurant biz, we used salt, it would absorb grease and oil and also scrubbing action would smooth it out.

When the knife damage was really bad, we would sand it down or take to a lumber yard and have them plane 1/16 or whatever it took.  The big ones we used a belt sander and afterwards, oiled it down with corn oil. let sit then wash it with dish soap to absorb excess oil.  

If you mean cleaning it between uses, bleach is the best way to eliminate bacteria. Rinse and dry. Once it's too full of cut grooves, you're best to sand it to minimize the bacteria-attracting imperfections.

Another option is to use a stainless steel board. It'll flatten your sharp knife blades in no time but real easy to wipe clean.

Just have it plane down for fast result or sand it down slower and more time comsuming