Secrets

Much to the surprise of many of my friends, when I went to college I joined a fraternity.  No, I was never abused.  I didn't even drink at the time.  Some of the things I went through would probably be defined as hazing, indeed the brothers would joke about some events as such, but never did I compromise my morals or find myself in physical danger.  Much later I recognized that many of these "pledge events" utilized team-building techniques popular in corporate culture.  If you want to suggest that we drank more, then you clearly haven't worked for the right corporations.  Anyway, once this team is built, you really don't want outsiders claiming membership (for a variety of reasons).  My fraternity is a national organization.  How does one recognize one's own if you've never met?  I won't tell you exactly, but we can cover the principles.
 
Secret symbols: There is certain iconography that is associated with secret organizations.  They generally represent a specific philosophy and are usually emblematic about how the organization views itself.  While the full import of these symbols may not be understood by the general public, they do not tend to be important secrets, and the symbols themselves are not secret.  These symbols are too iconic.  If you see an eye in a pyramid, you recognize it.  When you want to use them, simple symbols are best, used together (like a code) or in conjunction with some other form of sign (physical, auditory, whatever).

Code words: Like symbols, these phrases require simplicity.  It is essential that these phrases remain unobtrusive to the people around you as well as the person you're speaking them to, if they are not members of the same organization.  They should be universal.  You don't want anyone on the outside to know you're looking for a coded phrase.  You can ask a question, or make an observation that would seem perfectly normal to anyone on the outside, but (naturally) their response would not be correct.  The correct response should be a little odd, but connected to the intro phrase.  An additional trait is that the intro phrase should be easy to disengage from.  If you ask directions and then don't follow them (because it's the wrong person), that individual might get suspicious.
                            example:  "Have you ever been to Samarkand?"
                                             "Yes, by way of Belgrade."
Both are perfectly reasonable.  No one listening would be suspicious.  An outsider could give the same answer, but it is extremely unlikely.

Coded gestures: Gestures are often the introductory code.  If I rub my earlobe, you rub your hands together, or something.  Mirroring is often the preferred method (I wiggle my nose, you wiggle yours, kind of thing, though perhaps something a bit more natural).  Following this you may move on to the coded phrases.  The most famous coded gestures are secret handshakes.  These are not elaborate things, like those devised by children or athletes.  They are subtle and easily disguised by those doing the shaking.  Generally this is the final confirmation.  It would be awkward to try to give a secret handshake to someone who didn't know it (especially a stranger).

The trick with secret societies is that they want to be able to function out it the bright sunlight with lots of people around.  Clandestine meetings are all well and good, but dark alleys tend to be populated with all manner of ruffians.  The favorite example these days is Hydra (thank you Marvel). How would it be possible for this organization to function with so many secret members?  Well, it is pretty unlikely that they could keep that big a secret for so long, but these are pretty simple (almost foolproof) methods for assuring someone's membership in your secret organization.
    Now you just need to ensure that no one ever shares the secret.  Traditional secret societies have levels of membership, ensuring that only the most committed are entrusted with the deepest secrets of the organization (this is the main problem with the Hydra thing, since one of their biggest secrets is that they exist, which all of the members apparently knew).  Those on the basic levels don't want to tell because the exclusivity of the organization is one of the essential draws (much like religion), they might also not realize that they know a secret anyone cares about.  When you have deeper levels of commitment, these levels will have their own distinct signs and symbols known only to member of those levels.
     Creating secret organizations can be extremely enjoyable.  It makes me wonder about those who founded some of those traditional orders (Masons, Elks, fraternities, etc) and created all of their signs and symbols.  Did they sit around with their friends thinking, "oh man, it'd be so cool if..." or was it all extremely serious as they assembled their symbology.  I hope they had fun.  Otherwise it all seems  too bizarre.