I'm seeking a list of commonly used abbreviations for US cities (and also European cities, but let's start with the US). When I say abbreviations, that could be anything from 2 letters or more that is colloquially used to represent the city, so I would include say "LA" as an abbreviation for "Los Angeles", "NYC" for "New York City", "VEGAS" for "LAS VEGAS"...etc. It doesn't have to be restricted to 3 letters like an airport code.
Explanation: In the iPhone application that I'm building, I'm wanting to create some precomposed tweet templates which the user may choose from to start with, so that they may tweet about a certain thing that has just happened in the app. I want to combine a word representing the main subject of the app or what they've just done (e.g. "fitness") with an abbreviation for the city they're in or their nearest city, and possibly have this term as a twitter hashtag.
So, for example, if they're in New York, the precomposed tweet template might contain the hashtag #nycfitness, or #fitnessnyc .... and I need to come up with the "nyc" part for whichever city they're in. Thus far, Google has found me lots of state abbreviations, and even city airport codes...but I don't think airport codes are commonly used to represent the city in many cases, e.g. people don't use LAX to mean Los Angeles...LAX is the airport and LA is the city. I'm not from the US, so while I can come up with a few of the big ones (LA, NYC, DC, VEGAS), I don't have enough local knowledge to compile the list myself.
Edit (provided here as edit rather than reply to reply because of letter count):
Ok, I understand perfectly the need to maintain the credibility of individual stackexchange forums as a source of truth on their particular subject. And whilst I provided some context for my question by explaining to you the reason for my requirement, this explanation was more for folks to get a sense of what sort of suggestions might be appropriate in response; it certainly isn't your place to use that context as reasoning for your condemnation - it's kind of none of your business what I do with the information I glean from this answer, and I've only given you that information because I thought it might get me more appropriate responses.
An example of an intelligent and helpful responder who took the information provided in the question and suggested an idea I hadn't thought of (the very purpose of these forums, one might think) was that provided by @J.R. regarding colloquialisms. If folks are likely to use the hashtag #beantownfitness, then clearly (because I've told you why I need this information) I'm as interested in colloquialisms as I am in abbreviations. Striking the discussion off on the basis that "This is a question about standard abbreviations..." is being a bit lazy, in my opinion.