Mexico is a country in Central 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.
Even if you don't ask people and stick out your thumb you can make 500 km a day if you are serious. Don't take drugs or weapons with you, because you likely may be checked by one of the numerous military posts especially in the north. In some mid-sized cities like tampico, oaxaca, pachuca there are no transit roads and you have to cross the city. Taking a bus makes things easier. The Guia Roji road atlas with the maps of the major cities is maybe worth the 100Pesos.
The best way to hitchhike in Mexico is to go to the petrol stations and talk with the customers (and even if you Spanish 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.
In rural parts of Mexico it's common to see whole families hitchhiking together, or for a pick-up to stop for several different groups of hitchers until the back of the truck is completely full. Because pick-up trucks are the vehicle of choice, it's quite easy to get rides just outside of any small town (knock on the back window when you're ready to get out). Occasionally waits are long, due more to local traffic than reticence towards picking up hitchhikers. On some heavily touristed routes with poor public transportation, payment is expected, though it shouldn't be more than 20 pesos (about $2) at most.
Hitchhiking is an easy alternative to taking the train, and occasionally the only way to travel between small communities through which a bus may pass once daily, or less. In small towns, ask around at stores if anyone's headed up the mountain that day.