Arriaga is a city in Chiapas, Mexico. If you are traveling around Chiapas you will very likely at some point end up in Arriaga. Although there is not much in the town itself, it's an important point of junction between many highways. From Arriaga you can head South to Tonalá, Tapachula, and Guatemala; North to Oaxaca, Veracruz and Mexico City; or East towards Tuxtla and San Cristobal.
"La Bestia" is the train in which many illegal immigrants from Central America travel, and it passes through Arriaga. There is an immigrant shelter in the town and its common for people to ask to see your papers when you're hitchhiking in Chiapas. Specially when you're hitching North.
There are many gas stations around Arriaga and you can easily sleep in any of them. The one right outside in the North is particularly good. It has a covered camping area (really useful during the rainy season), free showers and even outlets.
Be aware that if you're travelling between Oaxaca and Tuxtla/San Cristobal, there is a short-cut that doesn't take you through Arriaga. Many people use it, so you might decline a ride to Arriaga.
West towards Oaxaca
Take a colectivo (6 pesos) heading toward either Zapata or La Gloria. These pass some gas stations just outside of town. In July 2013, there was also a military checkpoint just across the state border in Oaxaca. You can ask the driver to let you out near there; it's about 1 km walk.
Nomadwiki & Trashwiki
Check Nomadwiki for info on accommodation, showers etc. or Trashwiki for dumpsters...and share your wisdom :)