I used the Global Entry System that I had signed up for on my last entry into the US. Immigration was so backed up I would have missed my connection if not for GES. I highly recommend it for US citizens who travel outside the US.
http://www.cbp.gov/xp/cgov/newsroom/publications/travel/globalentry_infoguide.xmlPS: sorry, I was reading the requirments and realize this would not be available to many of you. ;)"No person will be eligible for this pilot if they have been convicted of a criminal offense in any country; if they have ever been found in violation of the customs or immigration laws of the U.S. or any serious criminal offense; or provide false or incomplete information on their application. Additionally, no person will be eligible if they are determined to present a potential risk for terrorism, criminality, smuggling, or unlawful immigration."
AmbergrisCaye.com 
