I would say simply pick a really specific way to change the language, and then change it that way consistently.
Like with your example, it uses words that rhyme with the word they mean in English, in order to essentially create a new language.
Pig Latin takes the first part of a word in English, puts it at the end and adds "-ay" as a suffix.
So I would take a language, which could be English, and modify it in order to create a new one. As long is it's consistent, it should be decipherable.
It's essentially a form of verbal encryption when you think about it. With Pig Latin, you have the the word (for example "amscray"), and then the key to decode it (take the "-ay" off the end, then put the last part as the first part), and you have the word "scram".
So think of a specific "key" in order for the characters to be able to decipher each others language, let's say taking the important words and adding the word "in" in the middle, and swapping the beginning and end.
Then your characters can say "Is the pon-in-wea den-in-hi inside the uck-in-tru?" Which would translate to "Is the weapon hidden inside the truck?"
To expand my answer, it could be a good jumping off point for having an 'in' language. Eventually, common words will just be changed for some reason or other.
For example, using the rule for changing words that I said, eventually someone might create a new weapon and call it a 'ponin', a shortened version of 'pon-in-wea'.
People will only ever create their own language to serve a purpose. Whether that purpose is to befuddle outsiders, to distinguish members of their group or simply to make things easier, such as getting rid of superfluous words in sentences (think of words like 'don't' as a shortened version of 'do not'), the equation is the same.
Purpose + time = new dialect.
People come up with new words all the time for some reason or other, whether or not they catch on is part of the time portion of the equation. Like you say you want people to identify the in-crowd by the language they use, every single generation of teenagers goes through this exact same ritual.
Kids now apparently (in the UK) describe someone old and creepy as 'a bit UKIP', which is taken directly from the current cultural background, and possibly a misunderstanding of adult things.
So whichever rule you use, you need to think about the time element, and whether the words would realistically catch on. As for where they come from, take anything that people are aware of at the time of its creation and use it for your own ends.
As a footnote, another good example I like is 'Devil's Advocate', which originated from the Catholic Church as a person who argued against the side of God, and took a skeptical view of someone when discussions were ongoing about appointing them as a saint. This has now pervaded into popular culture and is a term commonly used by most.