(Particularly Kham and Amdo, where we've spent most of our time, aka parts of Yunnan Sichuan Gansu and Qinghai)
(also feedback is welcome, help me fill this out)
Where I've been and am most qualified to give advice about: Kunming, Lijiang, Tiger Leaping Gorge, Feilai si, Shangri-la/Zhongdian, Yubeng, Daocheng, Yading, Litang, Kangding, Chengdu, Langmusi, Xiahe/Labrang, Tongren/Rebkong/Rongwo, Xining, Kanbula, Zhangye, Lanzhou
Learn Chinese! This trip was more language-intensive than I think anywhere else I've been. If you're in Germany, say, everyone knows English. In a place like Korea, a lot of signs are English and you can usually app your way around. But China's a mega culture of its own, and while there's a lot of English signage, and many English speakers, I think it'd be really hard without knowing any Chinese.
I did Pimsleur lessons 1-90, which were mostly good. Practice listening and speaking is crucial, because it's what you'll do the most. Their context is kind of dumb - they always assume you're an American businessman in China to do business - but it's maybe 2/3 useful and 1/3 business fluff. I also tried this app Skrittr, which was good for learning characters - not as crucial as the speaking, but very useful to at least be able to identify maybe 50 or 100 characters.
Phone: get a phone! Or, a sim card. Or, get Project Fi which has reasonable prices overseas. (Maybe t mobile too?) This seems really essential. A common occurrence:
Me: huh, I'm at the address of the guesthouse, or at least close, where is it?
(Calls guesthouse, lots of Chinese ensues, which I don't understand, but I manage to say where I am)
(5 minutes pass)
Someone from the guesthouse: oh hi! Are you Mr. Dan? (Shows me phone convo record to authenticate himself) follow me!
(We go around a corner, into an alley, up an elevator, across a way, up another elevator to the 32nd floor, into someone's apartment that has been converted into a guesthouse.)
Seriously. Things are not so walk-in here.
The Land of Snows http://www.thelandofsnows.com
The Adventures of Jonas https://jcrimm.wordpress.com
These two are the two best English sources I've found for the whole region. Especially Land of Snows - everyone reads it, and Lobsang, the writer, can even answer questions if you email him. Jonas is more under the radar, but has lived in the area for a long time and writes really great bits about places to get you excited about places you didn't know much.
Other solid blogs that have helped me here and there:
China Nomads http://www.chinanomads.com
To Go Back https://togoback.wordpress.com
Bamboo Compass http://www.bamboocompass.com
ATMs: I think the only ones that work with foreign cards are Bank of China, ICBC, and China Construction Bank. ATMs from these big 3 are in all big, medium, and small cities, but not very-small cities. So Feilai si, Yubeng, Litang, and Langmusi were lacking them, the rest had them.
WeChat (it's the Chinese SMS, WhatsApp, and Facebook. Everyone is on it. Easier than phoning even.)
Ctrip (for booking trains)
Booking.com (for hotels, better than Ctrip's selection)
Airbnb (it's getting pretty popular; often guesthouses just use it to advertise, but whatever, you found a place to sleep.)
Uber (works here! even better: if you have the Chinese characters for where you're going, you can copy paste them into the Uber app, and then you don't have to explain to the driver where you're going)
Pleco (draw a Chinese character and it can tell you what it is)
Google Translate (and download the offline Chinese language pack) - this does photo translation too which is occasionally super baller. It also does draw-a-character but whatever I like Pleco.
Maps.me (best offline maps I've found, though I've heard good things about Galileo too. That's a caveat - it uses OpenStreetMap, which is still not quite Google Maps quality, but getting surprisingly close! Make sure to download the China provinces you're visiting for offline access)
Baidu Maps (all Chinese, but useful for copy-pasting an address sometimes)
A VPN or three - usually the VPN provider will have their own app.
Anything else you might need! China blocking Google means that the Google Play Store won't work unless you're on VPN. So make sure to download all your apps just in case your VPN isn't working.
Other online helps: China DIY Travel helped me book a train at one point.
VPNs: you need at least one of these. China blocks all sorts of sites (incl Google, Facebook), but if you've got a VPN going, then instead of you asking for, say, google.com, you ask some rando computer in the US, then that computer asks for google.com, and sends the website back to you. China doesn't see you asking for google.com, it sees you asking for some rando computer in the US, so it doesn't block you. Sounds complicated, but most VPNs make it pretty easy to set up. The thing is, it's one more thing to fail, and when you're dealing with crummy wifi and poor Edge connections, one more failure sucks. So I have two VPNs (Privateinternetaccess and OpenVPN) and sometimes one works better than the other. Sign up for them (pay them money, usually like $6-10/month), and make sure you get them fully set up before you go - like actually connect to them at least once. If you've got a VPN for your university library or something, don't count on it - turns out my CMU VPN only VPNs traffic to paywalled journals/conferences. Which is useful, but not enough for me in China.
Watch for gouging: on tourist goods and taxi rides. That's the only places I've ever felt ripped off or like you need to bargain. Bargain for tourist stuff, and either agree to a price beforehand or use a meter in a taxi. (Or use Uber!)
Books: two books I really enjoyed were Country Driving by Peter Hessler (about China in 2010ish) and Trespassers on the Roof of the World by Peter Hopkirk (about Tibet in the 1800s)