Upon arrival at the Mexico City Airport, and at other airports and major bus stations, there is a system of authorized taxis set up to help ensure the safety of travelers. Although they may cost a bit more than a cab you can hail on the street, it's a good idea to always take authorized taxis when available (the price is still very reasonable).
How to take an authorized taxi
First locate the authorized taxi booth or stand. These are usually marked by the sign "Taxis Autorisados" or in airports "Transporte Terrestre". There may be taxi drivers standing around trying to solicit your business. You should avoid these and head to the stand to purchase your ticket.
At the taxi booth you will see a map of the city marked off in zones and the cost for transportation depending on which zone is your destination. Tell the ticket agent your destination and pay the fare. This fare is for up to four people with up to two bags per person. If there are more than four people in your party or your luggage will not fit in a sedan then you will have to pay more for transportation in a larger vehicle.
After purchasing your taxi ticket proceed to the taxi area - you should see signs with arrows pointing you in the right direction. There you will give the ticket to your driver who will help you load your luggage into the car. Tell the driver your destination - and off you go. Generally a tip is given to the driver who helps you with your luggage, otherwise no tip is necessary.