Ok, I'm in babble mode this morning -
Neologisms – When you introduce new words, or old words used in a new way, the reader will be looking for internal definitions. It is a really good idea to put one in. Although it is true that not fulfilling this reader expectation can be used to increase tension, you have to do it very, very well because if you screw it up the reader will stop reading. The point where new words are introduced is the point where readers most often decide to abandon a story.
Slang Back Stories- Use your created slang to tell part of the back story. Figure out the cultural impact of the back story. Make up words and usage that reflects that impact. Choose parts of the history your readers most need to understand to highlight in this way.
Grammar – Don't put all your language change into slang. Spend some time considering changes to grammar. Pull out your favorite grammar handbook (Harbrace or Transitive Vampire or
http://englishplus.com/grammar/gsdeluxe.htm or whatever – You DO have a grammar, don't you?) and go through it, writing out examples of how the people in your story would phrase things as opposed to current usage. Keep in mind the effect of contrast in grammatical styles.
Example: A totalitarian society might be reflected in an overwhelming use of passive voice, sprinkled with characters who speak in commands where questions would be used in current American usage.
Example: The cascading structure of multiple prepositions is the hallmark of relaxed, casual, seductive conversation in an otherwise succinct business culture.
Narrative and dialog – Decide whether to use your lingual shift for both narrative and dialog or only dialog. Stick to that decision. I suppose there could be some case where the dialog is in currant usage and the narrative is shifted, but I can't think of any examples. It is sometimes easier to explain things to the reader if the narrative is in currant usage. However, if the narrative is strictly from the viewpoint of one of the characters then you pretty much have to use the upgraded linguistics in both.
Always keep your audience in mind. No matter how cool your lingual shift is, if the reader has no clue what is going on they aren't going to continue reading.
(Edited because Shecky is better at grammar than I am.)