1.) Understand that you cannot prearrange for a taxi to come and pick you up. So get out on that sidewalk.
2.) Know which taxis are available. All NYC taxis have lights atop the roof. When the middle light is lit, that means that the cab is available. If all the lights are lit, that means that the cab is off-duty. If none of the lights are lit, that means that the cab is occupied.
3.) To hail a taxi, stick your arm out while you step off the curb. Don't get discouraged if an available taxi picks you up. There are thousands of cabs in the city and you will get a ride. It doesnt hurt to flash some cash in your hand either. Cash is the only accepted form of payment for a taxi in NYC.
4.) Know the rules. A taxi in NYC has a maximum occupancy of 4 passengers. Small children sitting on your lap don't count.
5.) Give the driver a tip.