How to Pack One Backpack for Six Months

I knew when I packed to move to China for half a year that I only wanted to bring one backpack, assuming that at some point I’d want to be mobile for backpacking. Plus, you can’t take the cheap buses to and from airports if you have to lug a lot of luggage– and I have a total and complete aversion to the cost of cabs, even when they are cheap in China. So, what should you bring, if you just bring one bag?