I've been thinking about doing this for a while–cab etiquette is one of the great mysteries of New York, and I think the whole world could use a refresher–but the whole thing was catalyzed when I couldn't come up with a quick and easy mnemonic for my brother-in-law to explain how he was to know if a cab was available or not. Now, it was a joke–we'd just had a great meal at Prune in NoHo, and he was asking me to come up with something in meter that rhymed–but I figure that just 'cause I suck at poetry doesn't mean that I can't create a handy, dandy guide to hailing a cab in the Big Apple.

So I'll start with the first question: how do you know if a cab's available?

Let's start with the basics. As anyone who's seen an American movie in the last sixty or so years knows, New York taxis–at least the ones you can hail on the street–are yellow. Let's have a picture:

This state is only relevant to you when you happen to be near a cab that's letting out its passengers. When an on-duty cab lets off its passengers near you, it's pretty much an invitation to take a ride–just remember that, no matter how ugly the weather is outside, you need to let the passengers get out before you enter the cab. (This sounds like common sense, but if you live in New York long enough, someone will eventually jump into the back seat while you're trying to calculate how much of a tip to leave, guaranteed.) But if a taxi's dropping off a fare and has its off-duty light on, it's almost impossible to change the driver's mind, no matter how persuasive you think you are.

http://weblogthatderekbuilt.blogspot.com/2008/03/how-to-hail-cab-in-new-york-part-i.html