Bring a gaosu gong lu (public highways) atlas. That's gonna be your Bible. The cheapest ones are at the gas stations (30-40 yuan). Since you'll probably need beforehand to get to any gas station, you can buy your atlas at any big bookstore around (60-100 yuan). Don't expect to get one in English or even bilingual. It wouldn't be practical either, since most roadsigns are in chinese. Buy one road atlas in chinese, it will be easier to get locate yourself, and to make yourself clear when you point out your destination to any driver.
Note that the highways (red and yellow in the map) are often new and fast (100-160Km/h), where as the second level roads (yellow in the map) may be really bad and a fatal error. Stick to highways if you want to travel fast.
Cars have the character from their province and a roman letter indicating the city in the province (A is always the capital). If you are in Anhui and see a 成 (Chengdu's "Cheng") with some numbers and an A, that's from Chengdu. These clues just tell you where the cars are from, but if they are outside their province, 50% chance they are traveling outside, 50% they are heading back home, so you can use it as a quite reliable critteria to select which cars to stop. Sometimes, 4 cities in the same highway have A the first one (capital), then B, C and D. If you go from the capital and wanna head far thru this highway, don't pick B, cos yea it may be going to the fourth city but most probably it will stop where it belongs, at the 2nd one. Etc.
Most of the entrance to the highways from the cities in China, have a toll. Go there and talk straight to the drivers, pointing the atlas. Show them your notice. Go like: "Nihao! Qing wen, ni qu nali? Wo qu zheli, ni ne? Ni qu zheli ma? Wo shi yi ge ren!" (Hello! Excuse me, where are you going? I go here, and you? Do you go here? I am just one person!), etc. Keep saying "Oh hao hao, xiexie a!" (Oh good good, thanks, ah!) Unlike in Europe, ppl there will typically tell you the truth about where are they going, and once they do so, if you ask them to go with them and they have free space, they will hardly say "no", either because they are just too naive or because they don't want to loose face.
At the tolls, if you talk to the booth girls and the guards and tell them where are you going and what is your purpose, they will often offer you to ask the drivers for you as they keep passing with their cars through the booths to pay the fee, so you can just sit down and wait.
Tolls also have toilets and rest areas for the personnel, where, like in most places in China, you can get in showing enough self confidence (people there hardly question the authority... or people seeming to have it). This, and the lack of malice (at an individual level at least!) of the Han, makes most of China easy to hitchhike 24h without fear. Folks over there are genuinely good people!!
If you want to sleep while on the road, just pick a slow and confortable truck, you'll make 400km in 8h but that's better than getting into a city (count all the time to get in and out + expenses) and even better than sleeping at the tolls or gas stations!
When no tolls around, go to the rest areas where you can talk to people easily, or look for a jiayou zhan, a gas station. Without a big sign, I never got picked up there with the passing cars (ofc most dont stop); with the sign, it took from a few minutes to seconds.
==Regions ==