From the Airport it is often easiest to take a car service into the city. These rates are slightly more than a regular Yellow Cab, but they are knowledgeable drivers, and if the taxi line is long you might save a lot of time.
You can also make reservations from these car services to get around town so you don't have to worry about finding a cab and braving the dangerous New York City streets. :)
For the Airport you call and make a reservation. Once you arrive and have your luggage, you call the car service number and they will tell you where to find the car. The cars can't park in the pick-up zone, so they drive by with the car number written on a placard, which they display in the window.
La Guardia $31