If you are traveling on a lower budget or if you want to experience the place like locals do, taking public transportation is a good way to get around.
Mexico City, Guadalajara, and Monterrey all have metro systems. Taking the metro is quick and very cheap. Another cheap way to get around is a city bus. The route is displayed on the window of the bus, but it´s always worth asking the driver when you´re boarding. They often have fixed stops, but in some places, you can hold your hand out to stop at any street corner. Be aware that public transportation in cities can get crowded, especially during rush hour in the bigger cities.
To travel between cities, Mexico has a good bus network, with comfortable, frequent, and reasonably priced bus services.