Most Chinese people dress for comfort, so you can plan to do the same. There's little risk of offending people with your dress: Westerners tend to attract attention regardless of their attire. Fashion capitals Hong Kong and Shanghai are the exceptions to the comfort rule: slop around in flip-flops and worn denims and you will feel that there's a neon "tourist" sign over your head. Opt for your smarter jeans or pants for sightseeing there.
Sturdy, comfortable walking shoes are a must: go for closed shoes over sandals, as dust, rain and toe-stomping crowds make them impractical. Northern Chinese summers are dusty and baking hot, so slacks, capris, and sturdy shorts are best. A raincoat, especially a light Goretex one or a fold-up poncho, is useful for an onset of rainy weather, especially in Southern China. During the harsh winters, thermal long johns and thick socks are a lifesaver—especially in low-star hotel rooms.
That said, in urban centers you can prepare to be unprepared: big Chinese cities are a clothes shopper's paradise. If a bulky jacket's going to put you over the airline limit, buy one in China and leave it behind when you go. All the other woollies—and silkies, the local insulator of choice—you'll need go for a song, as do brand-name jackets. Scarves, gloves, and hats, all musts, are also easy to find.
Most good hotels have reliable overnight laundry services, though costs can rack up on a long trip. Look outside your hotel for cheaper laundries, and bring some concentrated travel detergent for small or delicate items. Note that it's often cheaper to buy things than have your own laundered, so if you're even a little interested in shopping, consider bringing an extra, foldable bag for carting purchases home.
Keep packets of tissues and antibacterial hand wipes in your day pack—paper isn't a feature of many Chinese restrooms, and you often can't buy it in smaller towns. A small flashlight with extra batteries is also useful. The brands in Chinese pharmacies are limited, so take adequate stocks of your potions and lotions, feminine-hygiene products (tampons are especially hard to find), and birth control. All of these things are easy to get in Hong Kong. You may also want to bring along a face mask to protect yourself from dust and pollution, especially in the smoggy winter months.
In your carry-on luggage, pack an extra pair of eyeglasses or contact lenses and enough of any medication you take to last a few days longer than the entire trip.
If you're planning a longer trip or will be using local tour guides, bring a few inexpensive items from your home country as gifts. Popular gifts are candy, T-shirts, and small cosmetic items such as lipstick and nail polish—double-check that none were made in China. Be wary about giving American magazines and books as gifts, as these can be considered propaganda and get your Chinese friends in trouble.