For those of you who haven't used it, Mexico has an excellent intercity bus system with many classes of service available to fit your needs.   It is becoming more usual for the main bus terminal (Central) in many cities to be outside the downtown area, and also for them to have a "taxi counter" inside the terminal where you purchase your taxi fare.   This is a blessed relief from having to beat off taxi drivers entreating you to use them, when you have no idea how much the ride should cost.

Second Class (Segunda Clase) 
The most basic is usually Second Class, which can be comfortable, without a/c and making stops at many points along the way (few have a/c).

First Class (Primera Clase) 
These buses are very nice, with reserved seats plus more leg room, with a/c, restroom, and movies. A sandwich and a drink is usually provided. They make few or no stops along the route. Between major cities there is often a bus every 15/30 minutes, and there is no need to book in advance except during busy holiday periods. However, many companies have online booking facilities.

Luxury Buses (De Lujo or Ejecutivo)
The top of the line are a few companies such as ETN, ADO-GL and UNO, who operate luxury buses with 2+1 seats across which almost lie flat, similar to business class airline seats. These buses run mainly on the busiest routes. They provide a snack with sandwich and cookie, and a permanent drink station in the back of the bus with hot water for tea and coffee. The buses make few or no stops along the route and on longer journeys they often travel overnight. Tickets can usually be purchased through their websites.

Purchasing Tickets
Each city has one or more bus terminals. Every region has several bus lines. You can find out about itineraries and purchase tickets in the terminal. You can also purchase tickets on major bus lines from any travel agent in Mexico. There are often "package deals" combining bus travel and hotel in most destinations with which a travel agent will assist you in finding what is best for you.

One of the best sites for bus travel on the lines operated by ADO, such as ADO,OCC, ADO-GL and UNO is www.ticketbus.com.mx , where you can see schedules, prices, seat maps and reserve seats. The entire country is not covered but it's one of the best sites for bus travel in Mexico.  Most bus companies are not listed on the ticketbus.com website, however - and there is no one source for schedule/reservation information.

Enjoy!