FOIA letter generator: http://www.rcfp.org/foia
Note that there's an art to writing an effective request. Too broad and you will be told 100,000 pages are responsive and how would you like to pay for the photocopying. Too narrow and you will get back "no responsive records found."
I think getting useful data on customs clearance would be a long shot -- good luck.
Every federal agency is required to designate a FOIA officer. In my limited exerperiance, the helpfulness of these officers vary wildly between agencies.
http://www.dhs.gov/foia-contact-information
TSA's is about 2/3 of the way down the page. Good luck!