Napoleon's advise is sound, but thre is another option that I use, especially when I have a lot of luggage:
When you arrive at the airport, after you have gone through customs and before you actually enter the terminal, there will be several desks offering taxi or "remis" service to the city. I ignore them , exit the terminal, turn right, and head the twenty meters or so to where the taxis are dropping off departing travelers. There I can get a radio taxi within a minute or two. In late April I paid 75 pesos and I was able to choose a decent taxi which was big enough for my four large suitcases...as opposed to finding myself in one of the beaters I might have ended up in if I had made the agreement inside the terminal. I agreed on a price with the driver in advance.
If you are new to the city, however, having someone sent by the hotel waiting for you when you arrive is probably worth the few extra dollars.