All visitors must fill out customs declaration forms to present on arrival. One copy of this form should be kept, to hand in to customs on departure. Each traveller is permitted to bring in four hundred cigarettes, two litres of alcohol, twenty fluid ounces of perfume and up to fifty grams of gold or silver. You can legally bring in or take out only RMB6000, and foreign currency over the value of US$5000 or equivalent must be declared.
Certain valuable items, such as video cameras, office machines, computers and gold should have import duty paid on them.
Prohibited imports include arms, ammunition and explosives; printed matter, film or tapes critical to the country; dangerous or narcotic drugs; infected animals, plants or foodstuffs. It is also forbidden to take out any of these items, or endangered species of animals or plants and antiques without export permits. You may be asked to show receipts for any cultural relics you plan to take out of the country.
Generally speaking, customs don't hassle foreigners too much. There are green channels and red channels. If you have something to declare, take the red channel. Otherwise choose the other!