Useful information and forms: 

Customs Clearance Forms:   https://forms.customs.gov/customsrf/getformharness.asp?formName=cf-3299-form.xft 
Power of Attorney: (...click here, will take a couple of seconds)
Supplemental declaration: (...click here, will take a couple of seconds)

LINKS

Associations:
 
www.lacmassoc.com       LACMA (Latin America and Caribbean International Movers Association)
 
www.barmovers.com        BAR  (British Association of Removers)
 
www.erc.org                    ERC (Employee Relocation Council)
 
www.paima.com              PAIMA (PanAmerican International Movers Association)
 
www.hhgfaa.org                HHGFAA (HouseHold Goods Forwarders Association of America)