Many countries have a guide to customs for their citizens that can make the process much easier. Always investigate the country you are going to yourself by inquiring at that country's customs office, but these brochures and sites might also be of service.

U.S. Citizens: The free pamphlet "Know Before You Go" at http://www.cbp.gov. Click on "Travel" and then click on "Know Before You Go! Online Brochure".
You can also contact:
U.S. Customs & Border Protection (CBP)
1300 Pennsylvania Ave.
Washington, DC 20229
Tel. 877/287-8667

Canadian Citizens: "I Declare", issued by the Canada Border Services Agency (tel. 800/461-9999 in Canada, or 204/983-3500 ) is helpful.

U.K. Citizens: HM Customs & Excise at tel. 0845/010-9000 (from outside the U.K., 020/8929-0152) can help.

Australian Citizens: A helpful brochure is available from Australian consulates or Customs offices called "Know Before You Go". Call the Australian Customs Service at tel. 1300/363-263, or log on to http://www.customs.gov.au.

New Zealand Citizens: A free pamphlet is available at New Zealand consulates and Customs offices: "New Zealand Customs Guide for Travellers, Notice no. 4".
For more information, contact:
New Zealand Customs; The Customhouse
17-21 Whitmore St.
Box 2218, Wellington
Tel. 04/473-6099 or 0800/428-786
Website: http://www.customs.govt.nz/default.htm

Japenese Citizens: Customs procedure in Chinese, Korean, Russian, French, Spanish, English and Japanese. http://www.customs.go.jp/zeikan/pamphlet/tetuzuki.pdf

EU Citizens: The EU offers helpful FAQ for customs in the EU at http://ec.europa.eu/taxation_customs/common/faq/customs/faq_1178_en.htm.