Step 1: Obish
Obish (or Obby Dobby, or Pig Greek) is a simple gibberish language game whos public knowledge seems to have faded. You can use this to your advantage, and have "private" conversations with people right in front of someone you wouldn't normally talk in front of.
Playing this "game" well will make your speech near-unintelligible to people who don't know the trick to understanding this method of speech. To people who understand, your speech will come in loud and clear!
Please, remember that this mode of speech isn't unknown, it's just not widely known. So conferring "privately" with someone in front of a car salesman (who will likely know all the secret languages) or involving truly serious matters is not recommended. There are some people know this language, so it's really just PGP (pretty good privacy), and a bit of fun for you and your friends or significant others.