In line with johnmce's answer, try to keep yourself in a little more compact space than you have to do in the US. Don't sprawl out every time you sit, and don't spread your things all about the car/table/floor/wherever. Basically, remember that close proximity is expected, so don't try to take up more than your fair share.
Talking loudly is more common in China than in other parts of Asia, so that's not quite as big a problem here as in other softer-spoken countries in the region.
Most importantly, I would think not coming across as looking down on the local culture is the main thing — as it would be wherever one would travel. Don't be overly critical of food, hygiene, politics (especially politics!), or cultural differences, and you'll be just fine.