I've always boiled them to loosen up the meat/gristle that is stuck on them. Then scrape them with a knife and/or your thumbnail.

BE CAREFUL NOT TO BOIL THEM TOO MUCH OR LET AL OF THE WATER BOIL AWAY. It will ruin them -- voice of experience.

An old Montana cowboy that I used to work with told me that if I wanted to bring out the color in a good set of bull ivories, would be to carry them in my pocket in a bag of chewing tobacco. It works! -- again, voice of experience.