PICK A FIELD THAT CAN'T BE VERIFIED. Try something like string theory or God's will: "I speak to God. I'm sorry that you can't also." Security experts are in this category: They have security clearances, we don't. We can't question the expertise of the NSA because we are not in the NSA.
CHOOSE A SUBJECT THAT'S ACTUALLY SECRET. Dan Brown invented a secret subject for The Da Vinci Code, so now he is forever an expert on this secret subject that no one can challenge. Anybody who attacks the secret subject is, by definition, part of the cabal.
GET YOUR OWN ENTRY IN AN ENCYCLOPEDIA. In the media age, everybody was famous for 15 minutes. In the Wikipedia age, everybody can be an expert in five minutes. Special bonus: You can edit your own entry to make yourself seem even smarter.