Hailing a taxi on the streets of New York City can be a tad intimidating for people who usually just call for a cab in advance. Do you really yell, "Taxi!" or whistle, just like in movies?
It's more important to stand somewhere where the driver can clearly see you and then stick your arm out deliberately than to whistle or yell. I'm sure those earsplitting whistles work, but I can't do one and I've never felt the need to learn.
There are, however, a few other things you need to know.
First, if it's raining, cabs are scarce. Leave extra time to find one, or take the subway instead. Also, while cabdrivers are required to take you anywhere you want in the five boroughs and Westchester County, they don't always like to do that. If it's close to the time their shift ends, they may stop, roll down their window and ask where you're going.
If you're in downtown Manhattan and say, "La Guardia," and the driver's shift is supposed to end in 20 minutes, he will most likely roll up the window and drive away. So, as soon as a cab stops for you, get in and sit down before announcing where you're going. They might make a stink, but once you're in the cab you can get the medallion number and report them if they refuse to take you to your destination.
Finally, if you're having trouble hailing a cab for any reason, go to the nearest nice department store or hotel. The doorman there can usually hook you up in relatively short order. Just remember to tip him a couple of bucks for his trouble.
