When your bus arrives at your destination for the day, resist the impulse to rush out of the bus, through the bus station and out onto the street. Hang on and check things out. You've got an opportunity to get some very useful information just by looking around. Of course your passport and money are in your inside pocket pouch and your single piece of luggage is securely carried in front of you.
First, watch the other passengers. They may exit opposite to the station and go out on a convenient pedestrian bridge, or walk out to the nearest low-cost transportation into el centro.
Next, check out the loading area. Each bus company has one or more spaces. Find the location of the buses that you'll use when you leave. You may notice that some companies have their own separate waiting room/ticket office. You may prefer the much cleaner restrooms provided by ETN luxury lines.
Now check out the main station. Find the ticket window that you'll use when you leave. Check the schedule and prices. If the ticket booths are not crowded, practice your Spanish and ask dumb questions such as, "Estarán llenos los autobuses que van para Puebla?" (Are the buses to Puebla going to be full?). See if you want to change your itinerary based on schedules, crowds or prices.
If you don't have hotel reservations, (I never do), find the public telephone sign. Most of the time there will be someone there who will place a call for you. Using your handy pad and pencil, just hand over the name and number of the hotel you want to call. This will cost 1 or 2 pesos, but its better than going to a hotel that's full. The lady will dial the number and when it's answered, she will point you to the booth. Even if you can only say, "Buenos días, Senor. Necesito una habitacíon," at least they'll know that you're on the way and you'll know that they're still in business.
Now may be a good time for coffee and a pastry. Snacks at the bus station are not the best, but they aren't the worst either. Look at your map. Get oriented. Organize your stuff. Check your guide book. If you were lucky, the telephone lady drew a map to your hotel and/or told you which local bus to catch. If you need a taxi, by the time you saunter out, the taxi drivers will see that you're travel-wise and they'll be more helpful and less aggressive.
From The People's Guide Travel Letter #4