Guangzhou is the third largest city in China.
West towards Nanning
There is a toll gate at 22.747377,108.271414 which is easy enough to get to and is good for getting a ride. Use Google Maps to find the metro and bus routes to take. Once there, a sign is useful because the spot is also an unofficial long distance bus stop and people will presume you're flagging a bus if just thumbing. We'd recommend only trying to get to Yunfu from this spot which will get you to the first petrol station on the toll way and from there, you can petrol station hop all the way to Nanning. Once we got a sign to Yunfu sorted, we got picked up within 10 minutes.
We camped right next to the toll gate in a little triangle shaped park. It has comfortable grass, and no one much cares. Just be careful of "landmines."
东湖收费站： Take the subway to 马鞍山公园 (Maanshan Gongyuan), and exit thru Exit C. Find the bus stop just a bit south east of the exit, and wait for bus 29. The bus leaves as early as 7:00, and probably late into the evening, but the times I know that are listed are: 9:00, 10:00, 11:00, 12:30, and 14:00. However, the bus did not leave on time, and I ended up taking a taxi for 15 RMB. The bus will take you to a strange on-ramp where all cars going onto the highway have to do a U-turn before going onto the ramp, making the base of the ramp a perfect spot for hitching. Interestingly, a few cars pulled over offering rides to Shenzhen before one took me all the way to 茶陵, a few hours Southeast of Changsha. There are a few good service stations on the highway along the way though, so if you get a ride only going part of the way and are in a hurry, you could almost certainly get a ride straight to Changsha (or farther) from one of the service stations. The 炎帝 service station is particularly touristy, so there may be more long distance rides passing through there.