Mexico is a country in North America. Hitching in Mexico is extremely easy. In many places, locals hitch to get home from the grocery store, etc. You'll ride in a the back of a lot of pick-ups, and many people will offer you food and drinks.
The best place to get a ride is not on the side of the road or an on-ramp, like in many other countries, but at petrol stations or exits from shopping centres.
The best way to hitch-hike in Mexico is to go to the petrol stations and talk with the custumers (and even if you spannish is good, try to speak with a foreign accent ;) ). Some petrol station in the north states don't allow people do that, but you can try to speak with the manager, it works sometime. Otherwise, just stay on the cashier of the petrol station, or at the door of the food store or anything else (they can't forbid you that), and ask.