1. Give it a humorous name. Mine is Boris, after the cartoon villain the Bullwinckle cartoon. You can even imagine a cartoonish accent, like that of the cartoon.
2. Give it five minutes a day. I learned this from a therapist. You sit down for five minutes and just listen to everything the critic has to say, then you ignore it for the next 23:55.
3. Don't argue with it. Realize that, by definition, this voice will only be critical. It's incapable of anything else. Don't argue against it, ever. Just say, Oh, that's Boris. If you engage with it you are giving it a kind of legitimacy that it doesn't really deserve. Even if what Boris says has elements of truth, that doesn't mean he's ever right. Listening to the inner critic is just a bad habit.